Agreed, because that would be absurd for a St Neots to London or a London to Stansted Airport ticket.
However neither ticket says "not via Peterborough" and as the tickets are being used for "one journey" what restriction prevents the use via Peterborough?
In essence are they still regarded as two tickets, or does the combination create a 'new' ticket?
To be fair, neither ticket says "not via Aberdeen" or "not via Birmingham" either - doesn't mean you can go via those stations.
Combining tickets is perfectly legitimate, but as OwlMan says you still need to travel on valid routes for those tickets. Neither of the tickets you hold permits travel via Peterborough. I'm afraid ticketing and routing is far from clear and simple, so if you're unfamiliar with the system then your confusion is completely understandable - luckily this forum is on hand to help
My advice for your journey is as follows:
- If you really want to go via Peterborough then purchase a St Neots to Stansted Airport ticket routed "NOT VIA LONDON".
- A cheaper option, although not via Peterborough, is to make use of your season ticket between St Neots and Hitchin, and then purchase a Hitchin to Stansted Airport ticket routed "NOT VIA LONDON" (incidentally the St Neots to Stansted Airport ticket routed "NOT VIA LONDON" is also valid this way, so you could travel via Peterborough and Ely one way, and return via Cambridge and Hitchin the other).
- Using your season ticket and then purchasing a London to Stansted Airport ticket (as you initially proposed) is also cheaper than a St Neots to Stansted Airport ticket and permits you to travel from St Neots to London, and then London to Stansted Airport.
- If you want the flexibility of going via Peterborough or via London, buy a St Neots to Stansted Airport ticket routed "+ANY PERMITTED". This is valid both ways, and also includes travel on the London Underground between Kings Cross and Liverpool Street if going that direction and if required.