I would go with the one 12 inch inline for the lights only and you will see a temp decrease as well. It won't be enough as far as exhaust.. Go with an 8 inch passive intake, and reduce the otherside of the 10 inch inline from 10 to 8 inch. So you will be running 10 feet of ten inch ducting then reducing and finish your run with the 8 inch. All the ducting should be on the exit side of your inline,with the exception of the peices you use to connect your string of lights.:thumbsup:

I am guessing your in a 10x10 or an 8x8?

Ga..