If both bikes have a frame which is set up for it, with the long drop-out on the Thorn pictured below and the EX Box below it, then it would be the work of 5 minutes. Just unscrew (possibly by hand) that big nut on the ex-box, remove the wheel and put it onto the other bike.
The only issue to remember is to always shift to the same gear (say, the lowest) before you remove the EX-box and ensure the shifter is in that position when you reconnect, otherwise you won't be able to get into all the gears.
If, on the other hand it's a more complicated set up with a reaction arm like this:
Then it might be more complicated. Then again, perhaps the arm can be disconnected with a single pin, I haven't tried it. The two cables under the black flexible plastic cable covers will also need to be disconnected. I'm not sure how hard that is.