If both people are living in the same household, then both should be able to be listed on the same car insurance policy, even if they are not married or not related.
In fact, most car insurance companies will require that you list all household members on your auto insurance policy. If you have a friend who has moved into your residence with you, you should notify your insurance company to add that person to your policy.
The policy will need to be in the name of the person who owns the vehicle. If you want to add not only the person to your policy but their car as well, then it may get tricky.
Most car insurance companies require that you have insurable interest in a car to insure it under your policy. If only one person owns both vehicles, or both of you and the friend are on the registration of both vehicles, it is normally fine to have two people, married or not, with their cars on the same policy.
Instead, if you and your friend each have your own car in your own name, then it will make it more difficult to find a car insurance policy that will cover both of your vehicles. Each of you would not have insurable interest in the other's vehicle, so insurance companies may not allow you to put the friend's car on your policy.
You may end up with two policies in the household. If that is the case, you may need to still list the friend as a driver on your vehicle (since you live in the same household), and the friend may also be required to list you on his or her policy.
If you are able to find a car insurance company that will allow both you and your friend's vehicles to be on the same policy, there should be a multi-car discount for your household. In general, this discount will make your one shared policy cheaper to have than two separate policies.
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