I have those same sawhorses as well. They were the cheapest available, and it shows.
I'd say be very,
very careful trying to clamp anything to them (or even to another sheet on top of them, as has already been mentioned.) I've noticed with heavier pieces of wood (and heavier tools) that those sawhorses will have actually have some movement/flex to them. Also, be sure you're on a stable, flat ground surface.
The last thing you need is your table shifting on you when you've got both hands on a router or circ saw rolling at full speed...
Seriously: be super careful, and/or get sturdier horses when you can.