Form with Subform

You can create as many forms as you like with PlanSo Forms. In your case you could create a basic form that transfers the user to a thank-you-page containing another form. This way only those users that have submitted the first form will see the second one. You could create a complete wizard using this method.

With our Pro Version you can additionally add conditions to the thank-you-page so that the form is shown only if one field has a certain value. I.e. a select box asking whether they want to register family members.