Hi,
So here are some questions/thoughts
1. I am only assuming User1, is submitting the document? You don't have any details at all so I am guessing
2. After user one submits, you would need a flow, that would grab the Uploaded Document, from the Upload Document Question, and copy/move
it to a location that the other User(2) can see it.
3. They would grab it from there and change it, then they would upload it.
Example
1. User 1 submits Foo.docx
2. the flow copies it to a SharePoint location for users to review documents, but the documents are Read-Only to the Users
3. User 2, downloads a copy of Foo.docx and makes changes.
4. They submit it using the Form
5. And the Flow once again grabs it, but this time it see's that the document already exists, and overwrites it.
As long as you have history on, you can see when it was modified.
And that answers how to do what you asked.