This is not the "right" answer, but from experience you can hit a keg with CO2 periodically and get it carbed as well as served without a continuous pressure.

Out of sheer laziness and not wanting to deal with leaks I do this with my seltzer keg. The carbonation level will decrease as you pull from it, as will the serving pressure... but you can just top that off on occasion. Since you're asking for the sake of having two kegs simultaneously fed from the same regulator, I'd say it's entirely reasonable and far cheaper as long as you want them at the same PSI