If there are four stations A-B-C-D, can I combine an A-C and B-D ticket if the train stops at both B and C, in the same way as an A-B and B-D or A-C and C-D?
I am assuming that you would not be leaving the station at either B or C, i.e. you would not be breaking your journey (and that we do not need to worry about whether break of journey, and therefore starting late/ending early, is restricted by either of your tickets).
In principle, there is no rule against having two tickets/paying twice for a train journey. So long as you can show a valid ticket for B-C, it doesn't matter that you have another valid ticket for B-C in your pocket/wallet/purse.
But Condition 19 of the NRCOC says:
You may use two or more tickets for one journey as long as together they cover the entire journey and one of the following applies:
....
(b) the train you are in calls at a station where you change from one ticket to another;
I would say that your A-C and B-D tickets together cover the entire journey. But does paragraph (b) apply? That strikes me as an interesting question, since you could change from one ticket to the other at B, at C, or at any point between them. And it is not obvious how, or whether, you can show where you are changing from one ticket to the other.