Minimum Clearances when drilling an overflow

[lastlight]

I think you need a few more inches between them otherwise the primary is going to be sucking air from the surface when it starts creating a vortex.

Not sure if this is an external or internal overflow but remember you can always drill a new hole from the side of an external or through the back wall into your internal and have it higher up so the top of the elbow/emergency is just below your overflow level. My water drops 1/2" into my overflow box since the water line is controlled by my emergency which always has a slight trickle going into it.