In Visual Basic, similar to other programming languages, you can open sessions to a number of GPIB devices with a single GPIB interface, however it's important to know that if you do so, you are still working on a single bus. So while it is possible to open multiple sessions, and to even multithread your application, in the end you are still working with just a single bus, and only one thing at a time can happen on that bus.
Are you looking for specifics of how to do this? Have you looked in the NI-488.2 Help? Have you looked at different examples that install with NI-488.2?
Logan S.