Mixing wavelengths to produce light?
You can't get frequencies out of a linear system that you didn't put in.  Since passive processes in nature are going to be linear, at least at the individual cycle level, I don't think there is anything that will emit a frequency that is some function of two input frequencies without some sort of active element.  That's not likely possible with today's technology at light frequencies.

Mixers inside radios do make a output frequencies as a function of two input frequencies, but that is at much much lower frequencies.

There are other physical processes that can convert energy from one wavelength to another, but these don't operate over some continuous range as a function of two inputs.  For example, phosphors can essentially convert high frequency light to lower frequency light, but the output frequencies are fixed as a function of electron energy levels in the material.  A incident beam of wide frequency range can heat a object, which then emits black body radiation based on its temperature regardless of the incident frequency.  But these things don't seem to be what you are talking about.