Random Oracle
Warlord
Yes, you can have a maximum of one combat unit and one non-combat unit in a tile. So you can't have two workers or even a worker and a settler in the same tile.
Thanks for the information. That's an interesting change to the gameplay then - no more stacking workers to produce improvements in one turn. Probably makes it a lot more difficult to do combat engineering by building roads just as you are about to attack.