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.