Any chance you could pick up a manifold and hook both kegs up at the same time? That way you could get one carbed and leave it hooked up at 12psi and hook up the other keg when it's ready and start carbing it while you begin drinking the other.
I often crank up the pressure right when I start carbing a keg, especially if I've allowed my supply to run dry, but trying to carb to 12 psi by blasting with 30+ psi, then un-hook, carb another keg in the same fashion, then rehook-up the first keg and drop the psi to serving pressure (what, 4psi?) just seems like a major PITA. Get a manifold, get 3/16" serving lines for each keg at around 10' long (you could start with 11 feet and trim back until you get it just right) then you can carb, store and serve your beer at 12psi.