quote: My question is, how do I split the plumbing lines to be accessible to both units now or do I have to run a completely new set of pipes for the shower?
answer: it depends on many factors. Here in NJ you need a 2in waste line for the shower alone!
quote: And can the drain lines be combined?
answer: it depends on many factors. Yes under certain circumstances. It's called wet venting where you use the vent of another fixture as the drain of this fixture. In most cases you would wet vent the tub using the vent of the shower. Pipe size matters alot when doing wet venting.
quote: Also, can I install the shower right next to the tub with a 2X6 wall space between them?
answer: yes you probably can!