A multi-car insurance policy just means that you have two or more vehicles in your policy. If you have more than one vehicle in the same direction, you can add them all to your policy. Some insurance companies may indicate that they must have a garage at the same address, but an actual garage is not required. It simply means that cars must be parked in the same direction or close to it.
Your personal auto policy with Progressive can cover you while you're driving a rental car, up to the extent of the coverage you have for your own vehicle. For example, if you have comprehensive coverage for your car, that protection will extend to most rental vehicles. Common law marriage is recognized in some states, and your car insurance coverage will work the same way as insurance for a married couple. Progressive and many other insurers offer multi-vehicle discounts for having multiple vehicles on a single policy.
You can generally add someone else's car to your policy if you also share the same car insurance policy, which normally also requires living together. Most insurance companies allow (and may require) you to add another driver to your car insurance policy if the person drives the insured cars regularly or shares the same permanent residence. If your spouse drives your car and damages it, it's normally covered if you add comprehensive and collision coverage to that car. Insurance companies have different rules about who can or cannot be added to an auto insurance policy, as well as how many drivers can be on the list.
If you're married and living in separate homes, that's fine and, in fact, it's necessary to have separate car insurance policies, since your vehicles are parked in different places. In addition, if your policy includes car rental reimbursement and your car is damaged in a covered accident, Progressive will pay the rent (up to a specified limit and term) while your own car is repaired. Most insurers will also allow you to share a single car insurance policy with someone if you live with that person. Many insurers, including Progressive, require that you add a spouse to your car insurance policy if you both live in the same household (unless your state gives you the option to exclude your spouse).
If your child goes to school and has their own vehicle, the car can normally remain on their multi-car insurance policy as long as the student continues to use their home as their primary address. For example, married couples who live together and share car insurance can add both of their cars to their policy, even if the vehicles aren't jointly owned by the couple. However, the vehicle must be regularly parked at your residence overnight for Progressive to insure it in your policy. You can pay for your car insurance policy by credit card, PayPal, online check, personal check, money order, or electronic funds transfer (EFT) from your checking account.