How to bind multiple model (2 Salesforce custom objects) into one screen to collect data.

Let me explain with one business use case:

Use case: We need to develop a custom “Submit Case Form” where we would like to ask the customer to enter Name, Email, Case Category (Feedback, Inquiry, Issue), Description and upload any attachment(s).

a. Email should be check in account object, if exist - Case record to be created under that Account. And attachments(s) to be created under that newly created case.

b. If Email address is not found, it  should create a new account record, a new case record under the account record and attachment(s) under the case record. 
For end user - the web form should look like as one form.

You can create as many relevant models as you need and add several field editors to the screen with the necessary fields exposed.  You can also use conditional rendering to display models to use for Acct and Case creation, if another model didn’t return rows or hide them if the Account model did return a row.  

Have you started the process, would you like me to look at what you have so far?  If so, Copy and Paste the XML and I can take a look.