Now to be honest...I haven't done this for awhile, and it was originally my ignorance that let me discover a cheaper way to buy coffee for two at Tim Horton's.
Order their largest coffee, whatever they call it, then when the new hire (and they are always new it seems), hands you the large cardboard cup - ask for two real cups as well. You can then sit down and split the large coffee with a friend. It's cheaper than ordering two regular sized coffees.
I stumbled upon this money saving trick, while driving alone across western Canada, where Tim Horton's are easy to find and open 24/7. I travelled with a thermos, had one cup of coffee in the restaurant and put the rest in a thermos... one for the road.