Wyoming and Jack,
Thanx, I totally understand now, I got confused there for a second.
Yes, in US once an FO upgrades he becomes the most junior CA, unless a more junior FO upgrades before due to meeting total time requirements or jet time requirements before the other guy/gal.
Everybody keeps their seniority slot in US, regardless of seat. Now upgrades will come to whoever has the seniority AND meets the minimum requirements first, once they're both captains, everything else they bid for (vacation, schedules...) is based on their original seniority date (date of hire or initial hire check ride usually).
Not too sure how different it is in FD...