As long as both racks are identical, you can use a single probe thermostat to heat both.
If the heat sources are different, you cannot use the single probe thermostat as the wattages are different. So if one rack had larger heat tape than the other, it's going to either: 1) make the smaller one hotter than the set temp or 2) the larger one cooler than the set temp.
If the racks are different, you can use two single probe thermostats or one thermostat that supports multiple probes and can regulate multiple sources
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