Propane is a liquid in the tank. There is vapor above the liquid that is used for burning. As you use the vapor ( gas ), more of the liquid turns to gas ( boils ) to fill the space.
The issue may be that your generator runs on liquid propane, taken it from the bottom of the tank. The generator converts it to gas.
If that's the case, it will draw all of the liquid out of the bottom of the generator tank, while the pressure stays the same.
Propane pressure only drops when the liquid is gone.
No liquid will move out the vapor valve from the full tank to the empty one and the generator will stop.
If your generator runs on vapor, then tieing them together can work. The tank supplying the vapor will go very low before it starts drawing from the second tank. Path of least resistance.
Take a look at the generator tank valve and see if its marked " Liquid "