Because of what it adds to production costs, the use of multiple covers for the one edition of a book is still pretty unusual. There are a few cases I can think of off-hand: Dirk Wittenborn's Fierce People was available in a male or female version (both animal-headed)...
..while one of Irvine Welsh's awful, awful books got a range of colour choices.
And then there are examples like the Harry Potter books, whereby an 'adult' cover is released at the same time as the children's version, so that grown-ups reading second-rate children's books in public can ineffectually hide their embarrassment.
But with print-on-demand technology, the extra printing cost of having two--or even 10--different covers disappears. Hence this new release from OR Books, which was pointed out to me by Will of the phenomenal A Journey Round My Skull. Each copy is printed as it is ordered, with the buyer's choice of cover.
As I say, there could easily be a choice of more than two. It also ought to be very easy to have a facility whereby buyers design and upload their own covers, whcih would then be printed and bound onto the final book. I don't know of anyone doing this yet, but surely it's only a matter of time.