Convergence

Convergence is defined as two people combining their powers to do something neither could do on their own. It may be that the ability itself changes, or it may be that its capabilities are simply enhanced.
Since the Great War, we have known that anyone with manipulatable power can perform a convergence (power levels between 50.00 and 499.99). Thus anyone who manifests abilities can, in theory, converge (though it is unknown if two Amalgams can converge with one another; see below). There have been no records of naered doing so, however, and since human form and physical touch are (as far as we know) required, it is likely impossible for them.
Convergence is between exactly two people. Human form and physical touch are, as previously noted, required. The two participants become "one" in mind and power. They must agree or the convergence will be unsuccessful.

Combination of Abilities

The primary effect of convergence is, as noted above, combining the abilities of two Paragons. The specific effect will depend greatly on the abilities of the two people involved.
Furthermore, this effect can be hard to predict. Take the convergence between Dissipators Duncan Colbreak Whitewave and Kyle Stelworth Blackburn as an example. Duncan's personal ability is mist: he manipulates water within the context of air. Kyle can create fire. When the two of them combine their powers in convergence, the mist gains a fiery aspect. In practice, this means that their boiling hot mist can scald human flesh and kill naered (whose bodies, of course, are made of mist) from the inside out.

Convergence Between Amalgams and Paragons

The Great War was the first recorded instance of Amalgam/Paragon convergence in the modern era, though this was implied by older primary sources (which didn't distinguish between Amalgams and Paragons in the same way that we do).
Fire-manipulating Paragon Kyle Blackburn and Amalgam Vincent Bowman were able to create much larger, more destructive fires. With the prominence and popularity of their convergence, many people think that this implies that Amalgam/Paragon convergence strengthens and enhances the ability of the Paragon. However, Bowman himself reports that his convergences always take on a fiery quality, even when paired with Duncan Colbreak, Alistair Colbreak, or other Dissipators who are now dead. Some scientists speculate that Vincent's convergences imitate Kyle's because Kyle was the first person Vincent converged with. I tend towards the rather controversial belief that Amalgams (and likely Dregs) have affinities of their own, invisible due to their lack of power carriers, and that coincidentally, Vincent's is fiery like Kyle's.

Other Potential Convergences

Convergence between two Amalgams has not been recorded in modern times. Furthermore, the distinction between Amalgams and Paragons in writing is relatively modern, dating as recently as Fletcher's Rebellion, so older primary sources are difficult to discern in this regard.
There are mixed records of Dreg convergence. Primary sources from the Crathlian Revolution suggest that the powerless "full humans" were capable of convergence with the powerful part-dragon rebels, but this property has not been reproduced in modern times. The question also remains as to whether one needs aignaoi blood to perform a convergence. There is no scientific evidence to call this into question, but I personally have my suspicions. My grandmother Aisha Al Anbar Evenbrook has reported convergence-like telepathy with my mother Ophelia Evenbrook Colbreak, though I suppose this could be due to their shared gifted blood.
Shapeshifting has never been achieved through convergence, though it was attempted during the Crathlian Revolution on the side of the aignaoithe. (Then again, the side of the shapeshifters had very little luck with convergence in general.) The changing of form from human to dragon is theorized to be impossible through convergence because human form is required for its use.

Written by Cordelia Colbreak Evenbrook

Who can converge?

Amalgams
Paragons
Shapeshifters

There are mixed records of Dreg convergence. It was recorded during the Crathlian Revolution but has not been achieved in modern times.

Requirements

Exactly two people
Human form
Physical touch (hands preferred)
Emotional connection
Agreement in usage
Aignaoi blood (debated) or manipulatable power (debated)

Effects

Combination of the two people's abilties (in most cases)
Temporary telepathic connection

Discovered by

Alden Ó Flaithrí and Móirín Ó Ciardha
Seanán son of Bébhionn and Naoise and his son Cathair (debated)

Telepathy

Convergence opens up a temporary telepathic connection, even for combined scores lower than 200. According to primary sources from the Crathlian Revolution and the Dark Age, it is easier for human beings who already have telepathy to converge because the telepathic connection more easily translates to a power connection. For unknown reasons, shapeshifters do not seem to have the same advantage and struggle more than two humans do on average to converge with one another.
The Dissipators (to use a well-known example) used the side effect of telepathic connection to combat the visions the naered projected into their heads. When they suspected that what they were experiencing was not real, or that one of their members was trapped within a vision, they converged, which allowed them to more effectively fight off the visions.
Due to the telepathic connection and the sensation of another person's power running alongside one's own, convergence affords the partners a level of synchronization and "oneness" that cannot otherwise be achieved in human form.

Hands and Methods

During a convergence, participants must never break physical contact. Usually this is achieved by grasping hands, and since hands are a hub for power, this choice makes convergences more effective.
Each participant has one binding arm and one working arm. The binding arm stays anchored to the other participant's binding arm, while the working arm utilizes the ability. Ambidextrous people have an advantage in convergences because both hands are "dominant."

• Dominant Binding - Dominant Binding: These convergences are very powerful due to the fact that more power flows through the dominant hand, but they are characterized by lack of control, caused by the use of a non-dominant working arm.
• Dominant Working - Dominant Working: These convergences are much more controlled, but noticeably less powerful than those involving dominant binding arms.
• Dominant Binding - Dominant Working: The balanced convergence. The partner with the dominant working arm has more control over the manifestation of the ability, while the partner with the non-dominant working arm has greater power to work with.

Discovery

Anyone who has attended a Harvest celebration can cite the husband and wife who discovered and weaponized convergence. In the first convergence, Alden Ó Flaithrí and Móirín Ó Ciardha created the Ridge, a mountain range on South Lautney, and saved their people from the shapeshifters who would have killed them. Convergence quickly became the greatest weapon of the humans and played a significant role in the shapeshifters' ultimate defeat.
However, The Diary of Seanán son of Bébhionn and Naoise contains an interesting passage describing a naer attack in which Cathair, one of Seanán's sons, grasps his father's hand and summons blue fire. This is the first time Cathair displays signs of power, and the fire in Seanán's hand turns blue as well. In passing, he comments, "It was as if our clasped hands tethered our powers together" (tran. Ceallachán, 918). Unsurprisingly in context, no research was done on this observation of Seanán's. His supervisors likely discounted it as a result of his lack of skill with his power, or an exaggeration on his part to further place himself and his children on the same level (this was in fact something he did, early on). Thus the first convergence is actually thought by some historians (including myself) to be between a young (not yet at his majority, so the equivalent of teenage) colonial shapeshifter and his twelve-year-old human son.