A household with two vehicles is really making two decisions, and they rarely have the same answer. One car may be the daily commuter with the higher mileage, while the other is a second car used for short trips and parked most days. Treat them as separate purchases, and do not let the newer or cheaper car set the pattern for the other.
Start with the car you plan to keep longest
A service contract only helps while the car is covered and the contract is running. If one vehicle is the one you expect to keep for years, its ownership horizon matters more than its current condition. Write down roughly how long each car will stay in the household, whether you plan to sell or trade it, and whether a sale would carry the contract with it. Transfer rules differ, so read them separately for each car.
Check each vehicle against the contract limits
Every contract sets its own limits on vehicle age and mileage at the time of purchase. Find those limits for each car before you talk to a seller, because one car may qualify easily while the other is already past a line. The published tables on this site list the limits in one place. Start with the main ranking page and follow the tables it links to.
Compare the repair exposure, not the sticker
Think about what each car would cost to keep running without a contract. A car driven every day on long highway trips carries a different repair exposure than one used for errands. Ask each provider which major components the contract covers for the model year you are considering, and ask whether the contract is tied to one vehicle or can move between the two.
Decide the order
If both cars qualify, start with the one whose failure would hurt the household most. If only one qualifies, the other may be better served by a dated maintenance record and a repair plan the owner funds directly. Write the decision down with the age and mileage of each car on the day you make it, so you can revisit the choice when one car changes roles.