In most cases, during an ArcToolbox session, only one tool can be open at a time.
Only one command at a time can be passed to the server. This specification is in place to eliminate re-entry errors with the ODE Arc Automation Server.
There is an exception to this. If the tool, or wizard, does not utilize an ArcInfo Workstation command, it can be opened multiple times.
Start other ArcToolbox sessions and open a tool from each session.