What wavelengths to combine to get 488nm? Is it possible to combine a blue and green lasers to get cyan? If I used 405nm and 532nm, would I get (405+532)/2= 468.5nm? Is this a correct calculation?
 A: The answer to your question is no.  Mixing a purplish (405 nm) and green (532 nm) laser does not produce a bluish beam at 468.5 nm.  If you were to shine both lasers into your eye (which you should never do), the resultant effect would look bluish though due to the physiology of the human eye.
However, it is possible to mix two wavelengths of light to produce a new wavelength with the use of nonlinear optical devices.  The technique is known as sum frequency generation in which the resultant laser beam has a frequency which is given by the sum of the two frequencies which you are pumping it with.  As a simple example for your case you could take a laser at 976 nm and convert it to 488 nm (remember that $f=c/\lambda$) in which case the technique is given the special name second harmonic generation.
As with many nonlinear phenomena, you have to drive these nonlinear devices very hard to get an appreciable amount of power at the new wavelength.  So, you would need to find a very intense laser or pair of lasers in order to get a reasonable amount of light at the new wavelength.  
A: Note You should never, ever look at a laser. It can cause irreversible damage to your eyes, including blindness. Now for your answer.
Yes-ish
It will appear to be something in between, as you can see from a color wheel. However, you should note that this would be the color you see if you reflected the beam off of something.
It should be noted, however, that some other sensors will not be fooled by this. If you were to use a prism, for example, you could separate out the individual laser beams. A true cyan laser wouldn't get separated out by a prism. It only looks cyan (or whatever color you want to call it) to use because it stimulates the correct receptors in our eyes, and our brains translate that to be that color, not a superimposition of the two. (This is how we get colors like pink.)
