[Radek]

You can join both guilds immediately. There is no conflict among them except the last quest given by Trebonius, the Archmage in Vivec. As a Telvanni, you won't like the quest at all. But you can bypass the quest and become Archmage of the Mages Guild still.

If you join both guilds, do the Ald'Ruhn quests given by Edwinna as the latest one. Edwinna can promote you to Archmage, too. That's the way of bypassing the Trebonius' quest (kill all Telvanni masters).