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)?
To be pedantic, an aircraft's MTOW is a function of the structural strength of the airframe and landing gear and doesn't depend on conditions. The Regulated Takeoff Weight is what depends on temperature, air density, headwinds, runway length, etc, and is driven by whether the plane can get airborne before running out of runway (with a lot of margin for contingencies.)
For many planes under good conditions (high density, high headwind) acceleration to takeoff speed isn't the limit, but only airframe strength. Under those conditions, you could tow significant extra weight. The existing fleet of older planes includes many that have had engine upgrades, so they can pull a lot more weight than their MTOW.
Runway length has to include room for a rejected takeoff, for when the engine loses power just before you get to rotation speed. I don't think it'd be fun to try to reject a takeoff with a heavily loaded cargo trailer behind you. The trailer must have its own brakes, but it sounds scary.
