Not sure, but I think there is probably a circuit breaker in there or some other means of overload protection.
A lot of guys are making parallel adapters like this. >>
I think the only downside to this method would be that you need to be positive that you are not overloading the generators, because they will keep trying to supply the load (unless it is a gross overload) and can shorten the life of the inverter system.
I saw where one guy wired the ground wires to the ground prongs of regular 3-prong plugs (nothing hooked up to the two flat blades) so he wouldn't have to mess with a screwdriver to hook up the ground wires to the regular grounding points. Thus, making a quick-connect ground for the two generators through the duplex outlets.
I just bought the Honda eu2000i Companion model, which has the 30A plug already in it, thus eliminating the clunky box. All I needed was the eu3000 parallel wires to hook them together.