Unfortunately there is no way to bring a portal user in part-way through a business process. However, if you get creative with an external database you could do more to link the two processes.

For example, everything with BP1 stays the same up to the point where the email is sent out to the public user.

BP2 is started by the portal users, but, you include the business process instance ID from BP1 as one of the variables.

Once the portal user submits the form, you initiate a Workflow that passes the submitted information into an external database using the instance ID from BP1 as the identifier.

Then, the rest of your process continues on in BP1.

You would need to some way to "pause" BP1, but you could do that with a timer and an exclusive gateway that loops back in on itself.

For example,

1. User submits Form 1
2. Email goes to portal user
3. Delay process for ___ hours/minutes/days
4. Initiate a workflow that checks the database
   • If the data is present, update your process variables, and delete the row
5. Gateway checks if a variable was populated
   • If it is empty, loop back to the delay again
   • If it is populated, continue to the next step

Another option would be to put the Workflow in a loop instead of delaying/looping in the business process, then you just set the business process to wait for the workflow to complete.

Form 2 would simply capture the anonymous user's data, then initiate a workflow that plugs the values into your helper table.

One other benefit you would get here is that you could set it up as a lookup source, put the data in during BP1, then all you would need in the URL for BP2 is the identifier so you can pull everything inserted during the initial stages of BP1.