What I do with my herpstat and multiple heaters is I let the heaters run on their own for an hour then take the temp on the bottom of the heaters and see how close together the temps come out. Then I tape the herpstat probe directly onto the hotter of the heaters (usually they won't be exactly the same, but within a couple degrees of each other) and do some playing around with it to see what temp the heat tape has to be at for the heat inside the tub to be right. Then I just leave it set to that temp and hook up whatever other heaters I need and they should remain within 2-3 degrees of eachother if they're the same brand.