Yep, I do it all the time!
It's two black QDs, and a length of beerline.
Purge the first keg, and put the regulator at 2-4 psi. Put the first black QD on the "out" of the 2.5 gallon keg, and put the second black QD on the "out" of the original keg. The beer will start to flow. Pull the pressure relief valve on the small keg every 10 seconds or so, when the beer slows.
What I like to do is put the smaller keg in a warmer spot on the side of the kegerator, so I can see the line of condensation so I know when it's getting full.