3 Answers

Read here>>http://www.differencebetween.com/difference-between-monochromatic-light-and-vs-coherent-light/

Any two frequencies added together yields the sum, difference and both original frequences. The light of two lasers can be similarly combined to produce a differences frequency that is dependent upon precise tuning to acheive a measurable difference harmonic. The sum harmonic is usually easier to find because of the power level spike. But the addition of two frequencies yields a power level that is dependent upon how close in frequency the two sources are to each other to produce an intense difference frequency. Basically, 5+7=12, 2, 5, 7 and I have forgotten the power level formula for this.    

Two monochromatic light sources produce a "bell curve" of resonant wavelengths, each generating higher and lower harmonic wavelengths, with the highest amplitude at the midpoint between the two originating wavelengths. 

Newtonian Diffraction differs from double-slit diffraction in that the two wavelengths must be within 15% of each other for diffraction to occur.  For a bit more detailed analysis, see the link below.

http://www.jstor.org/discover/10.2307/229974?uid=3739976&uid=2&uid=4&uid=3739256&sid=21103325725993