Hmm... It is technically a consortium or a coalition (depending on the freedom left to both parties), I'd guess (though it may also sound a bit modern... even sci-fi), or you could just use the plain old Alliance word.
other ideas: Pact, Union, League, ???
Were you asking for ideas on just the denomination of the group (sort of blablabla consortium or Pact of blablabla, filling the blas with lore-friendly terms), or did you look for a whole name?
You could also go in the way of magical jargon. If some sort of binding spell was used to ensure the goodwill and loyalty of both parties, you could use terms such as "Bind" or "Contract"? Just throwing ideas here.
While I'm at it, perhaps the term "Conjuration". Etymologically, it's simply a group of person or entities that swore (juro) together (con)... to protect their mutual interests?
Anyway, good luck with your name crafting, it is one of the hardest (and I find, most interesting) part of creation!