Let's see; 1/2 wavelength on channel 6 is 5.8 feet. On channel 2 that's 8.6'. That's a bare minimum for modest interaction between the antennas. The real question is can you live with some coupling between the antennas? With strong signals, LOS paths, and minimum reflections you can get by with closer antennas. You'd be best off separating the antennas horizontally and positioned so that they do not aim at each other.
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