Aion Riftshade

I want to open two clients so i can enter group dungeons by myself. Is that allowed?

Can't speak for classic, but on retail it's tolerated.

How would you do it legally? Without 3rd party software or vrm

Retail can start 2 clients by default. Otherwise use an old laptop or something you have lying around, aion runs on pretty much any potato as long as you don't go to siege with it.

I think you can't open 2 accounts at once on Classic... it keeps telling you the game is allready running and you need to close it to start another account..