next, probably very newbish, question. i will sometimes/oftentimes be making flights with my girlfriend. if i book two tickets on, say, a 10k mile trip, will i earn 20k miles? is it better to have her enroll in her own program? my gut feeling says i will get 20k miles and she shouldn't get her own if we're flying together.
Miles generally get credited to the person flying regardless of whether you paid for them or not. Some airlines (NH comes to mind) allow you to pool family miles but that's the only one I know of. So sign up your girlfriend so you can both earn miles.
It's usually possible to transfer miles from one program to another but again, usually airlines charge you $$ to do so.