More expensive I know, but in your situation I would be tempted to have two cradles, one permanently fitted to each bike, if only to save a lot of faffing about transferring the cradle. That would allow you to have the cradles permanently connected to power, either direct from the battery or alternatively on the BMW, by breaking into the circuit which normally supplies the BMW GPS cradle, assuming it is still live if you don't have the GPS prep package installed.
On the other hand, I guess you may just not want the cradle in place if you are not using it, in which case I can see that your idea of powering it from an AUX 12V socket would make it more easily removable.