When several vehicles are registered to the same number plate, this is called an exchangeable number plate.
Full insurance cover is in place for the vehicle that is duly equipped with these exchangeable number plates. For the vehicle(s) not bearing the exchangeable number plates, liability insurance and comprehensive insurance (casco) only cover damage that does not occur on a public road (e.g. on a private parking lot). In case of a liability claim, for which the insurer is required to pay according to the road traffic legislation, the insurer will reclaim the benefits he paid from the policyholder. You can usually choose the type of casco insurance for each vehicle individually; i.e. it is possible to have, for example, comprehensive insurance including collision (full casco) for one vehicle and without collision for the other (partial casco).
It is not yet possible to calculate premiums for exchangeable number plates on comparis.ch, as this option is generally not available on the insurance companies' online calculators.