Hi,
Since this is File to Proxy scenario, so you cannot create message mapping, therefore I will suggest you to use Extended Receiver Determination, wherein the OM will contain message mapping which will have just for reference any dummy structure on the source side, and File Name as a field on the target side. Now, create a JAVA UDF for fetching the file name through Dynamic Configuration and map the same to the target File name. In the target structure you can have additional two fields say X and Y. These fields will contain the part of the file names as you suggested in the post (via the use of substring node function).
Note: Please make sure that you declare a Global Container in the java udf.
This Global Container can be used to compare the two files with respect to their substring as well.
Also in the ID part, in order to get two interfaces, you can have two Config scenarios in one party and a common Config scenario with Extended Receiver determination for the other two interfaces where you will create a OM.
Thanks,
Souvik