i think u got the idea mixed around - it’s 2 UTH’s plugged into a powerstrip and then the powerstrip plugged into a thermostat.
tho it’s not the best set-up, it will work as long as the UTH are the same size/wattage and brand. just keep in mind that the Hyrdofarm Jumpstart still has 1 probe, so u will be measuring the temp of 1 UTH. i’m assuming u will be using the second UTH for a different tank/tub? so the second UTH will turn on/off per the temp of the first UTH with the probe. u will need to check the temps of the second UTH more often to make sure it's ideal.
and as KMG said, make sure both UTH's don't surpass the wattage limit/fail safe of the thermostat u have.