for doubling up on gels it depends. Normally we don't do that, but there have been times I've done that where we didn't have a certain
gel color we wanted but I found that by mixing two that we had it came out rather close. I'm not sure how much it would
effect it but you should look into how much that will darken your light. As another result if you use two of the same
gel colors it should darken the light a
bit and most likely make the color appear darker as well. I know for a dance of ours I used two dark purples instead of one becuase for w/e reason they were coming out very much as white light.
I'd say some of the best things you could do for a question like this is to simply take one of your lights, move it to where you can change the
gel w/o any worriest, and just
play around. Try mixing a blue and pink or a blue and red and see what kind of light you get and if you like it. Also experiment with your specific instruments and see what
effect doubling up on
gel has.
~Nick