You shouldn't need 2 UTH's for a tub that small. It would be best to start with one UTH and thermostat. See how it goes for a few days and then if you feel the need to add another, go ahead.
As far as the holes. I just used a soldering iron to punch mine with and space them out about 2 inches apart. My humidity was still a bit high so I added a second row for more ventilation. If you are going to use larger holes then obviously you won't need as many. It's basically a trial and error thing. It's best to start off with just a few holes and add as needed to get the humidity right...
Rob
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