Is it possible to use different versions of same language with different prefixes?
Yes you can.

Go to the languages settings page (admin/config/regional/language) and click on the Add Language link.

There you can select a preexisting language, but below of that list is a Custom Language option.

Fill that form with your new language settings and there you have your new language.

To start, I'd suggest to export (admin/config/regional/translate/export) your main language and import it (admin/config/regional/translate/import) in your new language. It will give you a translated new language. Then, you can go to the translate interface and update the texts that must be different.