As sawdust suggested, buy a portable one. You can get them at walmart for 20 some bucks. I got one and rigged it up to add to my compressor and be removed for portability. It also allows for easy filling without holding an air chuck on a valve for 5 minutes. I just quick couple, turn the ball valve, and walk away.
Buy a 1/4" threaded ball valve, 2x 1/4" tees, 3x 1/4" by 1" long(or so) nipples, 2x 1/4" male thread to hose barbs, some air hose, 2x hose clamps, and 2x quick connect coupling sets.
On each tank, remove the air guage, and install a nipple and the tee. Leave the hose and air chuck on the portable tank. Install the air guages in the tees. In the portable tank tee, install a nipple and the threaded female end of a female quick coupler. On the main tank tee, install a hose barb and the hose with another hose barb in the end of the hose and then add the other female quick coupler. Take the ball valve and screw in the threaded male ends of the male quick couplers in each side.
Closing the ball valve, as opposed to just using a threaded coupler, avoids the excess loss of air when disconnecting the tanks.