Trainline does that automatically.
Are you sure?
For example, London to Perth, 19th Oct, around 4pm.
Compare ticketsplit and trainline for the connection departing at 1610.
Both are sending you on exactly the same trains (changing at Glasgow) but ticketsplit is actually selling you these tickets:
Note that the Euston to Glasgow leg has a ticket split at Lancaster: this does not mean that you change or get off at Lancaster, just that you switch between two tickets at that point.
Trainline is offering you a conventional off-peak single (more flexible, but significantly more expensive)
Now, if you want to cheat Trainsplit out of its commission you can go and buy those three tickets yourself, in which case the total would be £86.20. Or you can buy from them, they add their commisson of about £13 and you pay the £99.37.
Trainline add on a "booking fee" of £1.99 to the ticket price of £174 - but at no benefit to you, because you can buy that £174 ticket from any train operator's website without any booking fee.
Trainline is not even offering a basic ticket split - for example, if I was looking at this journey myself, I would always check what the cost would be if I bought Euston to Glasgow and Glasgow to Perth separately, and it's very likely I could find an advance ticket for each leg and it would come to less that £174.
Trainsplit goes further, because it manages to find a non-intuitive split at Lancaster, and knows that you can stay on the same train.
Inverness to London is a journey I do quite often; there is one direct train a day. Last time I was buying a ticket, I think trainsplit offered me a sequence of 4 or 5 tickets, but bought together they would cover the whole of that 8hr journey, with me staying on that same one direct train all the way. I actually didn't go for that, because it only saved me a few pounds on a more conventional ticket, and one problem with these kinds of splits is that you might be given a different reserved seat for each leg, which is obviously not ideal if the train's likely to be a busy one.