Does it matter? I run a Ramsey FM10C and a Ramsey AM1. Please don't laugh I'm a newbie with limited funds. As funds produce themselves I will upgrade, right now I'm getting a kick out of house/ yardcasting +. Part 15 is freaking awesome.
Chance
Even though part 15 transmitters are low power, I think they should be separated from each other, even AM and FM transmitters, because part 15 transmitters are built in plastic cases that are not shielded.
Transmitters housed in metal cases, which are better shielded from outside influences, can be closer together.
My FM transmitters are about 10-feet from the AM transmitters, and the two AM transmitters are 35-feet from each other.
I would like to see more technical information on how to get the equipment closer together in a limited area, but it's not easy to design transmission systems that share antennas and shelf space.
I've never had any problems with separate AM and FM transmitters, even when the antennas were only a few feet apart. The frequencies are at opposite ends of the RF spectrum, practically speaking at least.
What I have had minor problems with was an AM transmitter interfering with my amateur radio receiver, particularly on 80 & 40 meters (the two antennas were about 10 feet apart and I could hear my Part 15 station down in the static of the amateur receiver). But the frequencies in use there were pretty close to one another (1570 KHz & 3960/7185 KHz).