If you want to see a map of both user’s observations in different colors, you can use the compare tool (https://www.inaturalist.org/observations/compare) and enter user_id=A and user_id=B (use actual usernames) in the query boxes. This still does not give you access to edit another user’s observation though.
If you want to limit the display to only observations in a project, you can add that to each query, for example: user_id=jdjohnson&project_id=10914