Led will not mix even of you have a reflector. They produce a specific wavelength. How will this change if 2 are side by side and reflected back? It may mix where the light ends up but it will not mix the wavelengths. If that were possible could put a 400NM next to a 600NM and the end result would be 500NM. Instead the only thing that will happen is you have 400NM and 600NM in the same spot.