well, ok i just tried to log onto the college website & get a presentation explaining the whole fucking lot, but it's shit & i can't so im gonna have to try & explain it on my own.
because the wavelength is so small (nano meters) it makes it extrmely difficult to make it do what you want it to do on a large scale. however in a small scale its easier (they use this superpositioning in CD players & DVD player) its hard on a large scale because; if u had a source of light, the light goes in all directions from that source, meaning that their are thousands & thousands of light waves going away from the source. when one hits a wall (no matter what it's made from) it will reflect back. a grow room would be like the worst place for this because there's loads of different rays bouncing all over the place & depending on how far they've traveled when they meet it can act constructive or destructive. because its such a large space & the waves are so small you have no idea how it will end up. im not clever enough to work that out!
also as the MH & HPS are different light spectrums, they have a different range of wavelengths. blue=MH & HPS=Red
red & blue are on opposite sides of the visible light spectrum, so they have very different wavelengths. from what i know they will not affect each other.

in answer to your question you would have to work out the dimensions of the room to the exact nano meter & work out every possible path the light could travelfrom both sources. in other words unless you were a mega genius it would be fucking hard to do accuratly.

btw don't go taking your light combinations out, cos some will interfere good & some bad & some in between. but in the end it's just like having 2 lights.

anything u dont follow, tell me & i'll try & explain it better, sorry i just got up & im a bit tired.