Generally speaking I wouldn't use a stopper-cover combo together with an offside trap. The reason is that an offside trap is usually used along with a high defensive line, to push the defence up the pitch and reduce the amount of space between the lines while pressing. If you have a covering defender you're essentially dropping one man deeper than the rest of the line, meaning you're creating space in front of him and when trying to catch strikers offside there's a chance your covering defender will be too late (since he's closer to your own goal) and he'll play the opponent onside. If you have a good covering defender this may not be a problem, but I usually set up my defences like this:
Attacking/high pressing football: a high defensive line, offside trap, both players on "defend"
Somewhere in between: a mid to high defensive line, no offside trap, a stopper/cover combo if you've got the right players, otherwise just "defend"
Defensive/countering play: a deep defensive line, definately no offside trap, defenders on defend (you can't have stoppers stepping out of the line right in front of your goal).
