I'm not sure what do you mean with "have them connect simultaneously". You must understand the GSX GPU is really mostly graphic and doesn't perform a real function because:
- Default airplanes at default airports have a "magic" ground power that is always provided by the airport, regardless if a GPU is connected or not.
- 3rd party airplanes using a custom electric system cannot be charged by GSX, and they have their own GPUs anyway.
So, if you just want to add a GPU to default airplanes as an extra vehicle just for eye-candy, call it from the GSX menu before boarding/deboarding, since it won't make any difference to the availability of the external power, for the reasons explained above.