1. No. If you want to play at the same time, then you'll need to two seperate accounts.

2. Sub jobs grant you the abilities of that job (two-hour excluded) up to your sub's current level (capped at 1/2 the main, as you know). It also adds boosts to statistics. I'm pretty sure that some subs reduce certain stats. I think melee jobs without MP actually lower the MP pool of a job in most circumstances. That's it in a nutshell.