RE: How close can transmitters be
YOU shouldn't have a problem, but there is a small chance that folks around you might. If enough signal strength from your buddies transmitter comes back down your antenna (the antenna receives as well as it transmits) it will mix with your transmitter signal, causing what's known as Intermod Distortion, or IMD. The IMD frequencies are on either side of your pair of frequencies, separated by the difference in yours.
For example - let's say you're on 24, and your buddy is on 26. IF there's enough signal to cause IMD, there will be smaller outputs on 22 and 28. Your receivers should never know the difference, but if someone on those channels flies close enough to your transmitters, there might be an issue.
That's probably what the other guy was talking about - there were articles in years past in the model mags about this, with some recommendations for how far apart you should be.