Your only option is to put the 2nd GX on a separate computer. The software can handle only one GX scanning at a time. And a computer cannot download the 2nd software.
Or have one port dedicated to scanning different people and running the results on one of the 3 open GX ports (if both are on the same computer). This will make it easier in not changing out the accessories on the GX.
For more details, please check the link: