Most times when you purchase a car, it will be insured by the owner. Insurance service providers differ on this but most allow one to drive a car insured by someone else.
Some comprehensive policies allow the main drivers to drive someone else’s car on a third party basis. But this varies between providers and may be subject to certain conditions – i.e. you may need to be over a certain age. In any case, it’s always important to check with your insurer.
Also, what’s worth bearing in mind is you’ll only have third party cover on the car, even though you have a comprehensive policy on your own car. This means if you were involved in a crash in the other car, you’d have to pay out for repairs.