As long as you keep the pressure in the keg you can disconnect the gas line and have no worries about your beer losing carbonation. If you draw a few pints or purge the excess gas, then yes, over time the beer will lose some carbonation. But it won't be instantaneous or anything. Also, don't confuse carbonation of the beer with serving pressure. If you're pouring pints from a keg that isn't hooked up to gas it will still pour for a bit, but it will begin to diminish soon as the pressure is released to pour those drinks, making it appear as if you don't have enough carbonation.
The best way to serve two kegs with a single gas line is to get a "tee" to split the line into two outlets. A barbed brass tee with the right diameter will do the job for maybe just a few bucks. Otherwise you could step up to a manifold or distributor, but if you're looking for something quick and cheap for next weekend a simple tee is your best bet.