I have resized the rear drop outs on a tandem before. It took quite a lot of force. I stripped it down to a bare frame and clamped it down to a large work bench at both bottom brackets and the top of each seat tube. I moved one stay at a time to make sure everything stayed centred. You may also have to bend the drop outs to get them parallel. So, yes it can be done but it is a bit more involved than just removing the rear wheel and then leaning on it with a 2x4.