I think there are a couple of holes in your problem here. The first list is your infopath form, and the second list is a task list you have manually created. (not the workflow tasklist that is created) Am I correct in assuming this?
Also I'm assuming the workflow you created is attached to the infopath form library. The workflow variable that is automatically created will be stored there. Someone can correct me if I'm wrong but I don't think the workflow variable will automatically associate itself with the second task list your creating an item in unless you make the association yourself.
Here is how I have been accomplishing this. First off you need to set the workflow variable you created to some unique identifier. The form library list item ID works great for this unless you have an existing ID that is in your infopath form. Either way create a workflow action in the first part that sets the taskID to this ID before you create the task list item. The next step then would be to create a field in your task list to store this variable. Do that in the task list settings on the sharepoint site and then come back to your workflow.
Now in the first step it's creating a task list item. You have the option to set some (if not all) of the fields values when you create the task list item. Set the new TaskID field you created equal to the workflow variable TaskID you set earlier in the workflow.
You should be able to test his at this point so go ahead and save your workflow. When you create a form and save it, a task list item should be created and the variable for TaskID should be in the list. Now when you create stage 2 of the workflow you can associate the ID of the form with the TaskID again to bring up the proper Task list item. I would use the update list item action at this point.
If you have any problems with this just post and we'll see if we can work through them.
If you're not part of the solution, there may be good money to be made in prolonging the problem.