If two identical radios are side by side and tuned to the same frequency, will they both pick up the signal at 100%, or will they wrestle for the same radio waves?
A laymans opinion on the challenge: Waves lose energy, and the exact placement of antennas will matter. I don't know what the mechanism is called, but we don't place wind turbines right next to each other. That is afaik because each turbine takes some of the energy out of a larger chunk of the wind-wave in an 'bubble' around it, so we place them with optimal distance according to efficiency of that mechanism. If I'm right the effect will probably be minimal.
Anyway, just a stab at an interesting thought..
Yep. It's called near field and far field in radio. In the far field you can approximate it as a beam from the transmitter, while in near field it's magnets and things can absolutely interact. You never want to put up a stand-alone antenna in the near field of something conductive. Those big tower antennas actually incorporate the ground as a critical part of their design, because of that and the non-negligible conductivity of ground water.