there are a couple of ways of going about this.
The least user friendly method would be to switch the gas port between the serving keg and conditioning keg every time you want to pour a beer. annoying to say the least.
the next way would be to by a splitter for your CO2 line. Then, you can run one line to each keg and both will be on gas. (grab a cheap picnic faucet and you can drink from the second one). this is much easier, but the second keg will come to pressure relatively slowly and be at the same pressure as the keg on tap.
The third method (and likely out of budget if not ready to buy another faucet, but I'm including for completeness) would be a dual regulator. This will allow you to have both beers on CO2 and each can be at a different pressure. this is really handy for force carbination. I put my new, uncarbed keg on 35PSI for about 36 hours then turn it down to 12PSI and purge it. then it is ready to serve in another day or two at the most.
Hope that helps!