That's a pile of questions, so I'll attempt to answer them as best I can. Keep in mind I'm only a week or so in, and playing very casually.

Each class is separate, to a certain extent. You have two tabs of skills for each class. One is a "general skills" tab, the other is a "class primary skills" tab. When I'm a Rogue, for example, I can access Rogue General, Rogue Class Specific, and Priest General abilities. When I'm running as a Priest, I can access Priest General, Priest Class Specific, and Rogue General.

Each class does level up separately. When you hit level 10 in your primary class, you are given a quest to go talk to your class leader, which puts you on the dual-class quest. Your second class (once you pick it - you can wait a bit, if you'd like) then starts at level 1, and you have to level it up. The nice thing is that you are given the choice of starting where you were, or heading somewhere new. If you head back to where you were, it opens up new quests you didn't get on your primary class. Not too many as yet, but I consider the game to still be a bit in progress. It's stable, but needs polish, work on translations, etc.

So, to sum, yes, you play as one or the other, but you have certain abilities from the other class (for example, my Rogue can heal herself and others and do some limited buffing, but all are weaker than a priest primary) and my Priest can melee a bit more effectively thanks to some Rogue skills.

Oh, and you change classes by going by your house and talking to the lady there. I also opted to purchase (yes, real cash) an armor rack and a weapons rack to make switching class gear extremely fast. Pop in, switch classes, hit two buttons to change gear, and I'm back out.

Hope that helps a bit?