You can have two policies on one car - in fact, there can be any number of policies on the same item.
However, where there are two or more policies of an indemnity nature covering the same thing, the common law principle of contribution applies, which is a corrolary of indemnity, whereby each insurer pays the rateable proportion of the loss. So, if there was a £900 loss, and insurer (a) premium was £100 and insurer (b) premium was £200, insurer (a) would pay £300 and insurer (b) would pay £600.
You do not have to own something to have an insurable interest in it. The acid test is whether the destruction of something will cause you a financial loss. This is why it is perfectly OK to insure, in your own name, a hire car.