So much wrong with the above...
When you mix two wavelengths with a dichro, you don't get a third unique wavelength as the result. You get a third color (as far as your eyes are concerned) but the wavelengths don't change. You may get a color that looks, to your eyes, like another wavelength (for example when mixing red and green to get yellow) or you may get something that looks, to your eyes, like a color that doesn't match any possible unique wavelength (like when mixing red and blue to get magenta).
Also, while gadget is kind of correct in that he named a tool that identifies wavelengths, that doesn't change the fact that if you mix 650nm and 532nm beams with a dicheo to produce a yellow beam, the spectrometer just sees 650nm and 532nm light, not some new wavelength in between.