Hi Joris,
I think you could create a lookup field to the Contact Entity and Account entity on the Posted Form record. You'd have to do the same for Posted Field so that the entity would be reachable for editing in the workflow. I have not tried this myself but I think this should work.
In a workflow on the Posted Field record -
Check if it's Posted Form - 2nd Contact field does not contain data.
If the form field is 2nd First Name
Create Contact
Update Contact - Value from 2nd First Name form field into First Name field on Contact
Update Posted Form - 2nd Contact with newly created contact
If Posted Form 2nd Contact does contain data
Update Posted Field - dynamically put the 2nd Contact in the Posted Field's 2nd Contact field
If form field is 2nd last name
Update 2nd contact - Last Name with 2nd Last Name field value
If form field is 2nd email
Update 2nd contact - Email Address with 2nd Email
If form field is 2nd phone
Update 2nd contact - Phone with 2nd Phone
You'd need to run this on all of the 2nd First Name form fields first. And then do the same thing for the Account as well. If you want this to run on creation of the form field you could put a wait statement for a few minutes before this if statement "If Posted Form 2nd Contact does contain data", and that should then make that statement true for the other fields.