I don't think this will work as you planned...
I have a roadyxt and bought the xpress kit also. The receiver does not fit exactly. You will have to transfer the little black plastic base piece also BUT the car kit with the roady has only 2 cables, the power and the fm antenna/transmitter. The xpress has 3 cables, the power, an antenna, and a third wire that attaches to the antenna directly.
As a home kit though,the roady will work with it if you remove the plastic cover from the base station that the xpress sits in.
You could install the car kit in the second car and use the xpress instead of the roady, as it will work with only the two cables attached in the first car, you will have to use the base thing with the xpress.
I dont like the xpress as much as the roady because it doesnt show the names of the songs being played on the other channels until you actually select them unlike the roady which you can scan the channels to see whats playing without interupting what you are listening to.
It was a simple matter to call xm and transfer the account from one radio to the other, and then back again once I decided I didnt like the xpress. Now I'm using the roady in the house and in the car.
I hope this helps