New to the Game... IPA & Root Beer Kegging problem

I just got my first batch of brew ready to go... taste like crap - trial and error right? but my problem goes deeper. I have a mini refrigerator that i converted to hold 2 corny kegs, 1 keg with beer and the other currently with root beer but how can I get my PSI on the root beer higher and not effect my beer? I have a Co2 tank with two values but only one gauge so its either 15psi here and 15 psi there. I cant or dont know how to regulate different levels to different kegs. Anyone got any ideas?
Dumb question #2
How long would it take root beer to carbonate at only 15/12 psi?

You need either a double primary reguator or a secondary regulator. You will want at least 30 psi for root beer.

Yeah, like JRems said you're going to need a double body regulator like this. The other thing you could do, although it seems like it would be a major PITA, would be to carb your beer to 12 psi (or whatever you normally use), then disconnect the gas line. Then you could up the gas to 30psi and hook up your soda keg. Every few beers you'll want to recharge the keg with CO2, but if you don't leave the 30psi CO2 on there you shouldn't horribly overcarb it.

Also, not having kegged soda I can't be sure, but I would wager at soda carb levels (30psi) you're going to want a good deal of liquid line to make sure you don't just get foam coming out of the faucet. Hopefully someone that's kegged soda can chime in, or a search of the site should turn up a post somewhere where its come up.