it depends on the wattage of the flexwatt. Different widths have different wattages. The helix has a 500 watt max limit. So add up the wattage per foot total and make sure you don't go over that. I typically wouldn't like to use a single helix on more than one stack. And if you want the cages to have the same temps, then they need to be identical cages, with identical amounts of flexwatt in them. Using a helix in a stack of 3 like my retic cages, you'd want to put the probe in the middle cage.