Who's watching the screener

[Jake80]

I had a situation the other day that I was unsure who had responsibility to watch. 2-person game. It seems the offensive team was moving the ball to one wing and running the other wing off a screener at the free throw line. Opposing coach was yelling early and often for an illegal screen. Now defense was very aggresive so trail was watching action as the dribbler moved the ball to the wing. Lots of action underneath so lead is watching that. Who should be watching the screen at the free throw line? I told the coach I would try to watch the screen and the couple of times I did the screener was moving but no contact occurred. So, no contact no foul even though the screener did cause the defense to go wide and thus the offensive team gained an advantage. Any suggestions would be appreciated and no we are not allowed to have another official (which is what I would prefer).