Error while creating multiple workflow instances

Hi, I am very new to workflow product and have problems getting my prototype project working. In the workflow i have created 2 tasks (task1 and task2) and each task has been assigned two different roles (RoleA and RoleB). Completion of first task starts the next task.
When user1 (user1/group1/RoleA) starts the workflow from the menu after logging into the inbox , user1’s inbox receives a task (task1). user1 completes the task1 and the next task (task2) appears on the other user’s (user2/group2/RoleB) inbox. Now until the other user (user2) completes the task created in his inbox, user1 cannot start another workflow thread. Sorry if i am using the wrong terminolgies but hopefully you understand the problem.
Here is the error message i see on the workflow server, when i try to create another workflow when the previous workflow has not been completed.

ProcessServer.0: 3/4/04 [07:39:48.646 PM]
ERROR: Exception in eventhandler: Duplicate start by event[0] detected

Help here is greatly appreciated. Thanks