I don't know if it's technically feasible, but a chainable power supply, like the ones commonly used in guitar pedal rigs, might be an ideal powering solution - especially if one owned both units.
http://www.guitarpedalstore.com/guitar-effects-pedals.asp/pid_939/productname_Godlyke-PA9-POWER-ALL-Universal-Daisy-Chain-Power-Supply-for-Effects-PedalsThen again, if these controllers should evolve and eventually support MIDI merge/through, it's possible that we might own, say, two MidiCommands, a MonoJoystick and even a third product that may come down the line from R&W in the future. If a customer insists on spending less for a power supply, then there's always your bog-standard Boss-style guitar adapter.
Just a suggestion, with an eye toward the future!