There's no easy way around it. I've asked for a number of different tickets from a guard by asking for one first and then having them walk off before I could get to asking for the second. I have also asked up front if I could buy more than one ticket, and was given a look like I was crazy because he must have thought for more than one person and there was only me in the whole carriage.
Say hi, I'd like more than one ticket please and ask for them together 99/100 unless you get Mr Grumpy this will be fine. No more complex to do than selling tickets to someone with and without a YP discount for example. If you ask for one ticket, I quote you a price, and you pay up, there's every chance I'd just clear off straight away especially if it was busy or a station was coming up.
