I have a ballroom rack. I fit the probe though the same hole at the bottom when the plug wires are. Placed the probe on the heat tape so the tub won't hit it.
I have not drilled any holes in the ballroom rack, other racks I have. If you do place hole in the tubs, do them on one end. That will give you the option of turning the tub around if you need more humidify.

You should be able to run both sets of heat tape on one plug and the other one for the next rack.
I guess this can leave you open to one strip of heat tape failing.