Installing two ePMP access points (2.4 ghz) next to each other.

I know there is some math to acomplish this but off the top of thier head does anyone know how much horizontal seperation do I need between two ePMP GPS access points running two different channels to avoid any isues with self interfearance.

We are installing a second access point and antenna on the side of a building where we already have one installed.

The more the better.  I do however have one site where I have an ePMP 2.4 AP about 4' above a UBNT 2.4 AP on the same pole at opposite ends of the spectrum and they coexist peacefully.  Both are fairly low throughput though, maybe 20Mbps in total during peak times.

There is math based on wavelength that will tell you how far I know that. I have done it many times for 900mhz but not for 2.4. This site does not have that much room to place the new antenna. I figure someone knows the number off the top of thier head. Otherwise I will have to look up the info tonight and do the math. If I do Ill post the info for others.

This is a little bit funny because in this case we are placing them next to each other instead of on top of the other.