Do you have a unique identifier to the records? How would you determine "sameness" of the records?
With a unique identifier, you can use that as the basis of the logical test for a First(Filter()) construction to get the record from the second list.
Another option that I haven't tested... Patch() accepts a record as the starting point... so if you know the data structures of your 2 lists are the same and the fields are all exactly the same... AND you know that the record exists in your second list if it is in your first, I think you can just Patch() the second list, providing the record from the first as the starting point, and then supplying your changes. (According to your description, you would be making the same Patch() statement to the record in the first list, too).
@TimRohr is absolutely correct in his suggestions. It sounds like you could use some help with understanding the principles behind relational databases. If you have some time, please review my posts in the Community blog regarding this topic starting with the one titled: Database Design Fundamentals and PowerApps: An Overview