I wouldn't go near license holders transmission towers. On them is much better lol, but usually beside them tweaks the interest from RSM on unlicensed gear. Some unlicensed transmission gear can be somewhat unclean, like boosters that create to much hash (wide spectrum noise during amplification).

Funny thing is, if you're a few feet away it can cause lots of interference even with good gear, but being on the same pole makes it much less likely to cause interference. If you ever want to setup two antenna's or more always use the same pole if possible, and same earth for ligtning strickes, as the potential between the two can also cause problems in rf signals being warped in pattern from the site. As far as stacking antenna's on the same pole, it's called vertical isolation. This setup is less likely to cause distortion (intermodulation problems) between each antenna/receiver/transmit system, as they transmit and radiate outwards from their vertical alignment, not directly through each other at full force.
Either that or have the polorization of each antenna opposite to each other. One vertical the other horizontal etc... put them on the same pole about 1 and a half wave lengths appart (so you can fit another antenna and a bit in between).

I don't know if any of this makes sense or is of any use to anyone building a wi-fi site, but it's an excilent rule of thumb especially with flea power gear like wi-fi, where you want as much clean signal as possble. Also 1M height on any size pole from small to large makes a huge difference. Try to get it off any roof as far as possible, the more room to raidate and the less reflective surface for multipath problems (signals arriving at the same place at different times) the better the reception and speed.

Gavin.