Best Practices question on processing 2 files, read or select?

I have 2 transaction files with similar data in them, same key,
different transactions, both with a $ amount.  I need to process both
files and get the amount data.  I was either going to read through the
customer list and then go to both files and get the desired data by
key, but I already have a program that uses embeded sql and builds a
cursor for 1 file, I would like to add the extra file to the cursor.
Its not clear to me if this is even possible, I dont want an inner
join, but an additional file to select from that will also add to the
cursor.

Thanks.