When I was a kid, my Dad and Grandpa dug 2 ponds with TO20's a subsoiler and a scoop. One would rip, the other would scoop driving forward, then drive to the dam area and dump. It seems the trick is to skim a layer without bogging the tractor, yet filling the scoop before running out of space.
The problem with digging a trench would be that the scoop isn't as wide as the wheels on the tractor. You are limited to the depth the lift arms will lower the scoop below the tires. Maybe someone else knows a way, but I believe the best you can do to get 2' deep with a scoop is to go wider at the top and narrow at the bottom.