We need more information. Like what is the structure of the fields you are copying etc.

For copying within the same form, it can be as simple as Duplicate Record and followed up with Duplicate Record Macro Broken.

Being in separate forms and sub forms may increase the difficulty a bit. It would depend on your comfort level with vba or macros.

But the next question that begs to be asked since we can't see exactly what you are trying to do:  Why are you trying to duplicate data instead of using a relational query to just pull the data into both sub forms?

Two options: 1) good old copy and paste; 2) add a command button to the source parent or sub form and write some VBA code to move the data to the target sub form. Writing such a copy routine isn't for the faint of heart.

(Notice I avoided asking why you have two sets of identical forms in the first place.)