I'm considering a dual head for "continuous" water changes. I'm not looking to have "ALL" my water change done this way, as I still need to do regular maintenance like blowing off the rocks and beating back the pests, but I would love to do some automatic changes in between my normal 2 week schedule, or possible move my normal 2 week to monthly.
The stenners are one of the few options I've found in a reasonable price range (with decent feedback) with dual heads, so there is less chance of problems with calibration issues... One motor, two heads is more likely to keep things in synch in my opinion, and less likely to be something I have to mess with... other than replacing tubing on a 6 month or more basis.