Their current implementation (or prototype?) actually uses a powered airplane instead of a glider, which has its engines on for takeoff and then switches them off during the gliding phase (https://www.aerolane.com/aerocart, "How planes fly with AC0 today"). This way it works of course, but I'm not convinced it would be possible to take off with a glider in tow when the lead plane is already near its MTOW (if you'd have to reduce the loading of the lead plane to be able to accomodate the glider, that would defeat the whole purpose of what you're trying to do)?