What overbooking (unfortunately and too often) means is that the airline has allowed too many people to pay for the same service. Someone has to miss out. At this point you'd have to concede that your right to a second seat is less significant than someone else's right to one seat.

The airline should pay compensation though, and for both seats even where the flier used one, as they were puchased with the intention of using them together and the contract is therefore broken.