Should I run my fan faster or slower to air cool two light fixtures?

Good day! In your situation you are best off slowing the fan. The air needs to move slowly enough to exchange with the heat. A bigger fan will of course move more air faster, but your problem is heat exchange within the reflector. You can play with the speed for optimization. It is important to realize the overall efficiency of the heat reduction will be predicated by the reflector’s design in regards to cooling capacity, how the air is designed to run through the fixture. If you find the fixture is not cooling well after your experimentation with speed, you may be able to increase the efficiency with a damper directing the air within the reflector. This will depend on the design inside the reflector.