I regularly travel between two stations using off-peak returns. Sometimes I am given a lift back and have an unused return portion. This means I have two return portions when I next travel as it still makes sends to buy a return.
It has thus dawned on me that in this case, I'd prefer separate PDFs for each ticket.
However, I then need to remember which one I've used it not used, so I don't accidentally reuse one twice. Very important if I don't pass barriers in use and have my tickets checked. Currently, I do this by moving orange tickets around in my wallet. How easy would it be to do that with e-tickets? I guess I could download them and move them between folders on my phone, as and when I use them. This still lends itself to single PDFs.
If using split tickets, it may be that you use up one ticket but not another, so again it might be useful to know this in some way.
You may also finish journey short but which to keep the remainder of e-ticket, in case you have a use for it later. So again you'd need away if telling you what's left to avoid confusion.
However, I appreciate why some would prefer one. So I guess a choice would be best.