Can superimposed (combined) wireless signals create a health hazard?
This is what happens when you have a standing wave combining with itself in a microwave to produce a "hot spot". However, that requires:


very powerful transmitter (800W microwave vs 2W cell phone or 0.25W wifi)
confined spaces into which all that power goes
stationary transmitter and reflectors
continuous transmission (both phones and wifi transmit in bursts)


So if you take a bunch of low power devices which deliberately try not to transmit at the same time (like cell phones and wifi), and put them in a not-radio-tight space where they're not exactly stationary, then you're never going to get a powerful constructive interference spot. I don't think you could make it happen even deliberately with micro-metre precision positioning of the devices.