I am asked every year Jerry. I was surprised at the question so just said insure all bikes for a person on each seat and also insure me for 5 in my car. Its strange that this question is not asked on car cover but maybe it will come one day.
On motorcycles, if a pillion gets injured, its likely to cost a huge amount. My Grandson was killed on a pillion by a rider doing 70 mph in a housing estate. Rider survived and banned 3 years. He was just having a lift to work that day