merge 2 files for updation

See more:

C#

i am creating a windows application. my friend is also helping me. he has done some coding and design from his part. both use same forms but in different computers. No lan connectivity. we have portable drives to share the project.we have 3 forms in the application now. all 3 forms are used by both of us. now we need to share the application to update the changes. so help us to merge the files with updating the new codes and retaining the old codes. the changes made in both the codes are to be updated in 1 form.