Remove the sanke check valve in the receiving keg coupler. Connect the beer out lines together (I simply shove a piece of tubing on two picnic taps to couple the lines)
Pressurize the full keg with Co2 while the receiving empty keg has the coupler handle up so not to receive CO2. Beer will flow from the pressurized keg to the unpressurized keg, you will need to vent the keg you are filling as it fills up, and likely have to add co2 to the keg you are draining.
Its not that difficult, but takes a few minutes longer than you would think..