The old XJ6 had two filler caps, one on each side of the vehicle as you can see. What if Whitestar has the same feature? Two identical 70 Amp sockets mounted on either side of the vehicle. Now the Mobile Charge System obviously has a cable that fits the Roadster and hopefully will fit Whitestar as well. Here's the idea.
Redesign the Mobile Charge System so that the cable which plugs into the Roadster is rated for 70 Amps AND can be removed from the Mobile Charger becoming a single 70 Amp cable with a Tesla-compatible plug at either end.
You park the Whitestar in the garage next to the 70 Amp Home Charging Unit and you park the Roadster next to the Whitestar furthest away from the charger. Plug the cable from the Home Charger into the Whitestar and use the spare charging socket on the other side of the Whitestar to run the 70 Amp cable borrowed from one of your Mobile Chargers into the Roadster.
You effectively daisy-chain the cars together. Obviously power losses may be an issue,
Whitestar acts as the intelligent negotiator between the Roadster and the Home Charger. Assuming both cars will have the same size of storage and both are empty, it will take a maximum of 7 hours to fully charge both (2x 3.5 hrs)
Obvious issues - added weight and electronics to Whitestar.
Daisy-chaining only possible if your garage is big enough.
Cable between Whitestar and Roadster must be long enough so you can park, get out of the car without bashing paintwork and still connect them together.
Rather like V2G, this also raises the possibilty of V2V:-
Mr Eberhard: "Hi honey, er... forgot to charge the Roadster, can you drive over and give me a charge?"
Mrs Eberhard "Again? I swear you only bought me this thing as a mobile fast charger for your own car! Ok, be with you in half an hour"