For an incubator that small, I'd get rid of the fan and just run the thermostat probe directly into one of the tubs. Set it to 89 and you should be good. Just use a good IR temp gun to check both egg boxes to make sure the temps are good. Make sure you check the temps of the substrate as the eggs themselves will be 3-4 degrees warmer than the surrounding air and substate.

Fans are usually used in larger incubators to help keep a constant temp throughout the entire incubator. I don't think yours is big enough to matter.

I run a very similiar setup and seems to work great.