can i heat 2 different sized tubs with one thermostat

if i have a few 16 qt tubs and a few 28 qt tubs on one thermostat will one get a lot hotter than the other?

Yes if you are using one heat source to heat both tubs they will both be the same temperature

I think you will need more than one thermostat, either way.

If you are planning on using both 16 qt tubs and 28 qt tubs on the same rack, and have the flexwatt wired as one piece all the way through the rack, then you would need a proportional thermostat to control the heat (with the probe placed on the hottest spot of flexwatt), as well as an extra on/off (ranco) thermostat to act as a failsafe (with the probe also placed on the flexwatt). You would have to set your proportional thermostat to 90-93 (your desired hot spot) and set your "failsafe" to about 97-100, so that if your heat tape goes to 97, it will shut everything off. I asked this exact same question a few weeks ago before I bought my rack, and this is what a bunch of people told me.

For this reason, it is adviseable to only use the same thermostat for the same enclosures with the same exact heating elements. In the end, you don't save much money on the first setup where you need the failsafe. If you are building your own rack, it is much better to just use two proportional thermostats, one for each type of tub. If you are planning on buying a rack with 15 and 28 qt tubs, then the failsafe thermostat way might be the way to go.