Jevco tends to rate for the highest rated rider, but as few members have pointed out, other companies might have different arrangements. That's why it's always best to call your agent/broker/company and find out for sure. I can imagine how parents might have their two cruiser bikes packaged together with their home and auto insurance, and then their kid gets an M2 - wouldn't be fair at all to force him to be principal rider.
Also just to be clear - it's the insurance companies that make the rules of who to list on a policy. There's no law here that states that someone in the household with a licence must be insured under a certain policy. As far as the legal system goes, if the car or bike have a valid pink slip (insurance), and you have a valid licence, and you are using the vehicle with consent from owner (i.e. not stolen) - you are "legally" ok in every way. BUT, should something happen on the road, and you need the insurance company to help you out - then it's a whole other story. Now the ball is in their court and if you didn't play by the rules they can make the claim process very difficult, not to mention ruining your insurance record and dragging you through court if it's a big claim.
General rule then is, if someone lives in your house and has a licence, the insurance company must be made aware of them. If they live elsewhere and might use the bike/car occasionally (occasionally in this case for me means 1 to 5 times a month), there shouldn't really be a reason to disclose them. Just always remember that regardless of who's behind the wheel - if a claim occurs, it goes on both the driver's and the vehicle owner's records - so always think twice about lending out your bike or car.