If you still have RAF time available and want to try other classes now is the time to level them up. Don't wait till you have leveled your first group to 80.. make some other toons that you've always wanted to play.

As for your origiinal question, leveling the 2 together is a good idea. You don't take that much of a hit in terms of split experience and collection quests aren't really as bad with just 2.