2 Humans and 3 Transport Controllers

Hi, I have 3 operations in a production line: Labelling, Filling and Boxing. Labelling and Filling have dedicated operators but I want these two operators to be able to operate the Boxing station when a certain capacity at the end of line buffer is reached.

I am able to connect the third transport controller to the two human operators via the interfaces button, but they will not carry out the Boxing task. Does anyone know how I can fix this/create a workaround?