Oxford & Oxford Parkway is one I'd like to see. But I expect that wouldn't be commercially acceptable.
Once the Government controls fares, so competition between Chiltern and GWR is ended (making it just a matter of connectivity), that's a distinct possibility.
In Oxford's case, you buy a ticket from the "furthest" one and start/end short if you need to, surely?
Depends which way you go.
Let's take for example a journey from Bletchley to somewhere that can be accessed via London or Tamworth/Birmingham on a +ANY PERMITTED ticket, of which there are plenty of places (basically anywhere to the east and to the west). If you go via Birmingham, you can get out at MKC on the way back if you want. If you go via London, you can't. The fare is the same for most or all such journeys for all three of the main MK stations. This is plain silly, and would be solved by a Milton Keynes group.
The same thing also exists for Ormskirk, Aughton Park and Town Green to places like Manchester and beyond, where you can go via any of Preston, the Burscoughs, Kirkby or Liverpool. It used to be the case that you'd rock up at the one of your choice, offer to pay the extra fare and be let off. The trouble is that with PFs and prosecutions that doesn't work any more, so it's really useful to have it officially allowed.