iOS 8 iPhone Cellular Calls on iPad/Mac with 2 iPhones sharing same iCloud account
Ok, figured it out myself. My first problem was that for some reason my phone hadn't activated properly with iMessage / FaceTime (my phone number was unchecked in the You can be reached by FaceTime At section in FaceTime settings). That required switching off FaceTime and iMessage and switching them back on again after a short wait so that they could activate properly, which they did.

Once that was done, my FaceTime settings (on my iPhone) had a new option: iPhone Mobile Calls:



My wife's phone also has that setting. Choosing which phone is used is just a question of only having this setting enabled on one phone at a time. I assume (all though I haven't tried it) that if both phones are enabled in this way, FaceTime on iPad or Mac gives you the choice of which phone/number to use. It wasn't giving me a choice earlier because there was only one phone enabled.