pipe flow

I'm getting ready to re plum my 180g system with a reeflo super dart 4300gph as my main return pump and feed to my Emperor Aquatics UV Sterilizers > 80 watt High Output UV Sterilizer. the output pipe on the pump is 1 1/2". I'm going to drill two 1" holes for bulk heads in the left and right corners of the tank. with that said will going from a 1 1/2" pipe to 2 1" pipes decrease or increase flow