Well that's exactly what I did last time at the Brighton game. I have my family's 3 tickets on my phone. I ordered 3 drinks and 3 food items, tapped the 3 tickets onto the reader and saw 3x £10 deducted, and then paid the balance.
I was also told by the staff there that every £10 deduction has to be on a seperate transaction. It was a common complaint. Despite standing there with people, they wouldn't allow group deducts on one transaction, it had to be individual. Obviously a nonsense and contributed to queue chaos - because it meant people had to individually go.
If you have loads of tickets on your phone then perhaps it's a workaround, but most people won't.
The staff seemed to be making it up as they go.