The problem here is that the airfoils on the electric aircraft will be generating all sorts of aerodynamic forces. Since the airplanes are secured together tightly this will affect how your airplane flies. That means positioning is crucial. This is a much different situation than a glider on tow. I would personally work out weights and moments on paper before trying to send this thing up. That way you can calculate exactly where AND at what angle the airplane should be mounted at. It's more complicated than simply hanging the airplane at the center of gravity. Is your disconnect system 100% foolproof? A partial disconnect will send both models plummeting to their demise.

How did you calculate the 25 mph stall speed?