I would use a SharePoint Designer workflow for this. Have a field in your form called status (doesn't have to show on the form) and first time the form is submitted, set the status to "Step 1" or something. Then, in the workflow, if the status is step 1, you can email the next person w/ the link to the form. When person 2 completes her info, set the status to Step 2 - workflow can use that to email the next person.
I usually add helper fields to check who has been emailed - so - if the workflow sends an email for the first step, I'll have an "email 1 sent' field in the list that I set to true- that way I can use multiple conditions on sending the email: Status = Step 1 & Email 1 Sent != yes.
Make sense?
If SharePoint Designer workflows are not an option, you can send email directly from your form - let me know if you need help with that.