If it works like FFXI, they would have to be on different service accounts.

Lets say you have two mogstation accounts: A and B
Lets also say you have two characters on each: 1 and 2

So! Mogstation account A-1 could not play with Mogstation account A-2.

However:

Mogstation account A-1 could play with Mog Station account B-1 or B-2.