The HLLAPI driver will only allow one application in addition to the emulator window connect to a given session at one time. Each application needs to disconnect from its session before another application can use it.
Try to make sure that two applications do not need to share a session. If that is impossible, you can have more than one application share a session by doing the following:
•Have an application release the session when the application is not using it.
•Have both applications communicate through DDE (or some other mechanism) to coordinate the use of the session.
•Use an additional 3270 server application.