If your not in a hurry for the next keg, just leave it at serving pressure to carbonate up until you need it. At serving pressure it's gassed up after about a week. This is how I do all my kegs these days.
Otherwise if you need it more urgent, just drop the gas line from your serving keg, you will still be able to pour a few beers. Then hit the keg that need to be carbonated with 3 bar for 24 hours.