I've suggested before that you buy a moped and keep it in a shed just to keep your NCB alive, but I think that someone said that it doesn't cross over between a bike and car anymore.
We're doing the same. We swap between us each year so we can both keep our 5 years NCD. I'll insure one year, she can insure the next. We share the driving 50/50 so I don't see anything illegal in this.
I swap alternative years with my OH, we both have bonus as we had several cars, now we have one car but dont wanna lose the bonus so swap each year to maintain it, and it means that theres no messing about with mirroring etc if we decide to get another car.