Same account, playing two characters in the same server at the same time is possible?

[Risvertasashi]

Sharing the game in such a way is against the TOS anyway.

Realistically a lot of family sharing (and even roommate sharing) happens and SE doesn't do anything to them. That clause is probably more for stopping things like powerlevelling services or otherwise paying someone to play your character.

But to Tairin: No, you'll need 2 accounts/subscriptions to play simultaneously.