Saving values from one interface to another

Hi! I´m trying to create a complete invoice in two steps:

Step 1) First interface in which the user enters headboard values (id, total price, supplier, etc). The data is saved in Record Type 1

Step 2) Second interface in which the user enters the invoice lines (editable grid). Also, I want to link some of the fields of the previous interface, such as the supplier name or the invoice id.  The data is saved in Record Type 2

I´m linking both interfaces in the following process model (I´m only showing the left part, in the right part I make some checks with logic doors and write the records to the database. The process without the invoice lines works correctly, now I´m just introducing the invoice lines to the process)

The problem I´m facing is that I´m not able to pass values directly from one interface to another, putting them on a text field. I´m trying to do it this way with a text field:

Display value: rule input which refers to the previous interface data uploaded (Record Type 1)

Save input to: rule input to save the values into the Record Type 2

I don´t know which are the best practises to link fields from different records and interfaces. Am I proposing it in a correct way? Should I use the relathionships I created between the records instead?

Thank you so much

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