So stressful :(

[The Serpent Merchant]

Sounds like you have 2 different wattage of flex watt in the racks or different sizes/brands. I have no problem heating 2 different aquariums with one T-stat but I used flexwatt fromthe same roll cut to exactly the same length. So I would rewire/flexwatt both racks and you should be fine as long as all flexwatt is the same brand and wattage.

The differences in the rack design/materials/size make it impossible to safely control both with 1 thermostat.