Thank you for your post!
You can add more observers to an active assessment, have the observers complete the assessment, and re-run the report without conflicting with the 2nd assessment. However, if the 2nd assessment has become active, this would no longer be possible. When the 2nd assessment becomes active the 1st assessment's status is changed, so it would no longer be accessible. Changing the status of either assessment will cause complications with the assessment's availability schedule.
Ultimately, if the 2nd assessment is not yet active then there will be no complications in adding more observers and re-running the report.
Please let me know if you have any questions. Have a great day!