Most vehicle owners who drive two vehicles year-round will insure the most cost-intensive vehicle as their secondary vehicle.
Which car you should specify as a second car depends on the type classes of the two vehicles. If the second car is larger and more expensive than the first one, it is worth talking to your insurer.
It may be more favourable to then insure the second car as the first vehicle so that the SF class can be transferred.
Many insurers offer a second vehicle discount, which can make it easier and cheaper to insure multiple vehicles.