I'm thinking you should go beer tap to beer tap seeing as though its carbonated. If you do the normal beer out to gas in (like w/ sanitizer), you would end up with a bubbly mess.
-Pressurize the the empty keg w/ lets say 10psi.
-Put gas on the full keg, but make sure its over 10psi.
-Make a short beer line with two beer taps on it.
-Put the beer tap on the full keg, then connect it to the empty keg.
-Beer will start flowing until the pressure in both kegs are equal.
-When it slows down, simply release some of the gas from the empty keg using a spare tap/screwdriver.
-Repeat until you start drawing foam/air off the (now empty) full keg.
The key here is to keep the entire system pressurized, with just a small low pressure differential in the keg you are looking to fill. If you push beer into a new keg thats got the lid removed, you'll end up with half a keg full of beer and the rest will be foam.
BTW, why move carbonated beer from one keg to another? Combining beer from two kegs or something?