Try putting a few holes in the tub lid over the warm side. As the hot air leaves it will draw in some cool, dry air.

Check the room to see if there's an HVAC vent affecting the tubs, there may be a lot more air circulating around one than the other even if they are side-by-side.

Finally, if the male is getting into his water dish he may be displacing enough so it overflows, which will crank up the humidity.