My wife was insured with Norwich Union and she bought a new car and was going to sell the old one.
We rang them up and they gave us a one month overlap where she could be insured on the old car for one more month (to enalbe her to sell it) and also the new one at the same time.
I would phone the post office again and this time dont say you want to be insured with 2 cars, just tell them you have a new car and want to sell the old car and see if they will do temporary cover till you sell the old one.
If they wont I would find another insurer.