I looked up York - Sheffield and it said York was not a valid station, so I looked up Leeds - Derby and some trains do have Advance Singles, however for a return trip this was only £1.10 cheaper than a walk-up Off Peak Return, which in turn is considerably more expensive than "splitting". It was also suggesting an Advance Single and Off Peak Single combination that was nearly 50% more than an Off Peak Return!
Therefore I strongly advise people to use caution when booking XC Advance Singles, as you may not be getting good value for money. The other bonus of "splitting" with walk-up tickets is that more of your money goes to other operators, thus giving less money to a TOC that acts in a way that can at best be described as not customer friendly.