Say I build a full-wavelength 15m quad loop installed permanently on the wall of my balcony. Then say that later I build a small transmitting loop for comparison purposes, for use on that same balcony at the same frequency. Will there be unwanted coupling between the two antennas? And, if so, what can I do to avoid it? Would I have to "detune" the quad loop somehow when I operate the small transmitting loop (magnetic loop)?
Yes, there will be coupling. This is the principle used in many advanced crystal sets, where separate tuned circuits are air-coupled. You could orient one loop at 90 degrees to the other, or detune it. Simply shorting the tuning cap would probably work.