I'm not a lawyer, but here is what I believe should be done.
- Keep the original copyright line (You should keep the original author's copyright since you are claiming that your contributions make a derivative work.)
- Keep the license information (It's still GPLv2 since it is a derivative work.)
- Mention where the original file was obtained. A URL to a Github repository with a specific revision would be helpful.
- Add your copyright information. Remember to include a date in your copyright.
- (Optional, but recommended) Give a brief summary of what you added/changed
Your understanding of shared copyright over the file is in line with my understanding. The original author owns the copyright over the unmodified (before you change it) file. You will own the copyright over your additions to the file.
In reality, the copyright might be "shared" as you make modifications to the original author's work. Say you insert one line of code in an existing function. Sure, you can claim that the one line of code is "yours", but it's much harder to prove a single line of code was "stolen" without the larger context of the entire function. It's easy for a single line of code to end up in multiple projects without any of the projects having stolen from the others.