that gets tricky. planer objects cannont be modified. one thing you can do is select the isoparm of the outer planer then shift select the iso of the inner planer and hit planer. this USUALLY makes a planer with the hole punched out in the middle, but 90% of the time something isnt lined up correctly for the planer to work. what you can do is select both the isoparms and loft them together to make a surface. much easier and less hassle.