Yes you can. All my racks have a 6x11 piece of flex-a-watt under each tub 17 on one stat.
You will have some difference but not much.
Then again I also have a "dead" tub that has a hot spot checker in it to see what the inside the tub temps are too.
Where do you put the "dead" tub? I have a spare tub and my rack is coming tomorrow so it would be good to do this!