Simple question - I just don't know if there is an answer.

Are the two guilds (and all four/more if they add any lateron) mutually exclusive with regard to membership, or could the same character join and do quests of both factions?

No can be in both.

Vampire and Lycan is only mutually exclusive skill lines.

Yep, like Pang said, they are not mutually exclusive at all. You can be apart of both the Mages and Fighters guild at the same time. The only things that are mutually exclusive is the Werewolf and Vampire skill lines.

Posting just because!

Pang and Lep are correct.

The only ones? But what about necromancers/mages?

(Thanks for confirming the fighter/mage nonexclusiveness though!)

There is no necromancy skill line in the game right now. Necromancers are apart of Mannimarco's group an we are not on good terms with him.

Indeed. Bro's before Necro's!