How to complete a task based on a form save action?

Is form #2 the task that gets kicked off when form #1 is completed? If so, you can wire the task directly into form #2 in the workflow so when the user opens it, the serial number (SN) parameter gets passed to the form. Your form can then execute action on that serial number when the user clicks save.

Thanks for feedback - butI am not clear on how to accomplish what you are suggesting.

screenshot shows the first form workflow where I am sending task with a link to next form.  How do I add the the serial number (SN) parameter to the link so when user opens form #2 the 1st task is completed. 

I think I misunderstood the issue originally, allow me to try again.

The first task won't be able to send any information about the next task, so I would remove that link from there. Instead, the "Form 2 task", which I assume kicks off when form #1 is done, should include the serial number as a parameter (it is the second icon from the top on the list of task configuration icons) as well as include it in the notification link if you like. Then, on the form, you can use that serial number in an "Action a worklist item" rule.