[quote=@Minder]
Regarding side-factions: If you wanted to say that they are actually are a multiversal agency, you could flesh out the TSAB as a UMMA-friendly side faction.
[/quote]
The point was more that they hate both sides so both sides have to tread carefully in their zone of influence to avoid starting a war, so SE/UMMA conflicts in TSAB space are a lot less combat and a lot more cloak-and-dagger than they otherwise would be.

Another could be a cult dedicated to the multiverse's Anti-Monitor equivalent, empowered by something that exists beyond the bounds of the multiverse and hellbent on creating a gateway for it to come through and eat everything. No matter how many you wipe out, there's always another cell, and their presence is the one thing that can consistently call a ceasefire between SE and UMMA forces, because come on there's not much point fighting over the multiverse if there's no multiverse to fight over, though I admit I'm suggesting this mostly because I'm a sucker for "people that hate one another team up against a worse threat" scenarios, like with the Tyranids or the Borg.