The lore surrounding Enantiomorphs is something I've never really been able to pick up. Could someone explain it to me?

Were there any others able to perform it successfully besides Tiber, Zurin, and Wulfharth?

The Enantiomorph is a Myth Pattern. Essentially, it's a Duality where you can barely tell one side from another, as they switch places. We normally characterise this as Rebel and King, and the Rebel becomes the King, and vice versa. Furthermore, there's a third individual involved in the pattern, known as the Observer or Witness. The Witness serves two important functions: the first is that the Witness can tell the Rebel from the King, and the second function is a result of that, as it is able to see the moment when the Rebel becomes the King, and the King becomes Rebel.

There are other things involved in the pattern but those are the basics.

And, yeah, there have been a lot of times this pattern has appeared in Tamriel's history. Convention, where Auriel slew Lorkhan via Trinimac, with Magnus watching and then fleeing Creation, is probably the most important invocation of the Enantiomorph.

Is re-enacting the original Enantiomorph at Convention what made Tiber, Wulfharth, and Zurin merge into the Talos Oversoul?