My first thought is that you have the two content types based on article object:
Each request would reference a customer id.
So at start of request form, check email address against #__cck_store_form_customer.email using Seblod's '4' 'Required/Validation'.
From there it depends on your skill level with jQuery and php etc.
If email matches have a 'load customer info' button which is really a save button.
Using beforestore get the data from #__cck_store_form_customer
and populate #_cck_store_form_request fields/storages.
The form will save and reload with the customer info.
Then carry on filling in request
Something like that anyway :)