Hello Arkyadeep,
If I am understanding your client's requirements correctly, they want you to compare the changes done by two different developers on the same file even before any of them publishes their respective changes. There is no way to do this unless the changes are moved to central repository where versions are maintained.
In your case, probably first you need to explain customer how version control is done in OutSystems and share the various options available for post publish code comparisons via Lifetime or Service Studio. Yet if they are not convinced, you can ask them if they are aware of any such tool which supports their requirement. If they suggest any tool, then you can study its working, and if you really find it supporting such a thing then an idea referencing the tool can be submitted with OutSystems to provide such feature.
Thanks,
Junaid