It is physically impossible to have a single SCSI card and have more than one transfer going on at a time. SCSI does not support multiple parallel data transfer. One entity controls the bus at a time. Once the target starts the data in phase to the controller, that target has control of the bus until it releases it. In InveStore’s case that will be a 64 KB transfer.
This is where the importance of "disconnect" comes into play. Ideally you want the device to disconnect so that other operations can then be started.
The two options are to either setup the library in TARGET MODE using multiple-SCSI adapters (one for each Bus), or to consider the HP Fibre Channel configuration.