Definitely yes! On the other hand, you'll want to be a little careful how you stack them. Because tasks take differing amounts of time, it is possible to waste some effort. For example, building a road takes three worker turns. Putting a stack of two workers on the job will get the job done in two turns, but wastes half a worker turn. Also, you'll want to pay attention to movement cost. Those two workers building two tiles of road could do it together as move-road-road-move-road-road, or they could split up, with one doing move-road-road-road-mine-mine and the other doing move-move-road-road-road-mine. In other words, the same six turns could get you two tiles of road -and- three worker turns toward some mines, instead of just building two tiles of road. Pay attention to the difference between regular workers and industrious workers as well...