Current situation here is my wife and I each have our own cars, and the 20 year old and 23 year old each have their own car too. My soon to be 16 year old gets his permit in November, but he wont be getting his own car anytime soon since he will share the car with the 20 year old (thats the oldest car in the house currently even if its in good shape). Also all 3 are boys = more expensive on insurance.
All cars are titled, registered and insured under my name with Allstate. All cars have full coverage even the ones which are a little older since they are in very good condition, with $1000 deductible in case of total loss, at least I get some money towards replacing them. I also have an umbrella policy to fill in any gaps
This might not be the best logic, but when the oldest started driving we decided the best approach was to keep it under my policy since it is pretty much the top tier of coverage, discounts etc... you can get based on the collective good driver history of myself and the wife. We also didnt want them driving 100% shitbox and draining their bank accounts fully just to keep it running.
Once the older ones started working and making enough money to cover costs, they pay their portion of the car expenses from my policy. AT some point the oldest one will need to go get his own policy but due to personal circumstances that I don't want to discuss here, it's not the right timing.
FYI - yes you assign a "primary vehicle" to each driver and the costs/discounts are associated to that car's driving pattern (i.e. only use for work or school less than 10 miles away).
Not sure any of that helps your decision
@qclabrat hopefully it offers some ideas.