Do you mean splitting the tickets outbound (and using advances where possible) but simply offering a through ticket in the other direction? I suppose that's possible.
No, I mean splitting both ways, but with one direction on advances and the other on flexible tickets like a saver half.
Eg yesterday I needed to be in London for about 0930. I could get good split ticket prices from Diss to London on fixed advance tickets. But the returns didn't work for me on advance tickets as I wasn't quite sure what time I'd travel.
Flip to "flexible" tickets. 2 issues, the inbound ticket was now more expensive because I was no longer seeing advances. Secondly, if I moved the departure time for the return later or earlier, some of my inbound options disappeared. This is because booked as a series of flexible return tickets, the split points necessarily have to be identical both in and out bound, and differing stopping patterns limit the combinations that work both ways.
If you book advance tickeys, the split points can be different because they are single tickets. What I'm suggesting is to all advances one way and flexible the other.
This would give 2 benefits:
1. More flexibility where required without sacrificing value for money on the leg where advances would be acceptable. Of course you need to be careful the tickets are actually valid if you stay from the planned itinerary, just as now if booking flexible tickets through trainsplit. But it means I have options other than cut short a meeting or buy a new walk up ticket to get home if the advance no longer works.
2. As the split points are now independent of each other, in the same way as they would if I were booking advances in and out bound, playing around with the departure time for one leg won't make options on the other leg disappear.