If the insurance companies have their way, neither policy, A car in tow should be covered by the tow vehicle, same as if it were a trailer, However it's never that easy.
Best plan is to have both vehicles with the same carrier and thus the question is answered easily.
I used to work for a tow company (For a couple of years) When a customer claimed we had damaged his car we told him to turn it in to HIS insurance.. Fact is by law, if we damaged it, we were liable and it should have been our insurance.. But what usually happens is that if there is more than one company involved,, You turn it into the company that covers the towed and they get together with the other company and work it out without you being involved.