there all suitable for multiple tubs whan your only talking 5 or so.. when you get into big racks, some might be more suited than others.. the diference is a thermostat that has a "on/off" function, and one that has a proportional function.
i use both, and both are OK, but i prefer the proportional stats.
back to your question...... its all in how the flexwatt/heat source is wired.
you can wire each tub indovidualy,,, plug a "power strip" into the stat, and then plug each individual piece of flexwatt into that,,,,,,,,or you can "daisie chain" the strips together and use only one plug (directly into the stat).
probe placement varies from who you ask.... i place mine in the middle tub area, taped to the flexwatt.
spooky