You'll need three accounts. For the sake of simplicity, I'll list the ones you can use on staging:
one to post with (testy)
two that have the same pseud
1. Log in as testy
2. Post > New Work
3. Fill in the required information
4. Post Without Preview
5. Add Chapter
6. Check "Add co-authors?"
7. Fill in the field with "testy3"
8. Fill in some chapter text
9. Post Without Preview
10. You'll be on the ambiguous pseud page, which will prompt, "There's more than one user with the pseud testy3. Please choose the one you want." Choose "testy3"
12. Give the preview the stink eye because it just says "by testy"
13. Press "Post" anyway
What should happen
The chapter should be posted as "by testy, testy3" and the work byline should also be by "testy, testy3." However, testy3 is added to neither the work nor the chapter.
The part that says "There's more than one user with the pseud testy3. Please choose the one you want" is actually a label for the select menu beneath it. When you click on it, it should move focus to the menu. It wasn't doing that because the HTML was wrong. I have fixed that.