This happens sometimes. Most airlines do first-come-first-serve for offline jumpers.
However...some regionals have clauses in their SOP stating that when jumping offline, a more senior pilot has priority. The problem here is that a regional's SOP is obviously not binding on another airline, so the only way for that sort of policy to work is that the junior pilot who checked in first would need to voluntarily allow the senior pilot who showed up late to take his place.
IMO, that is BS. Nice try on the part of some regional pilot groups to legislate the operations of other airlines
It is basically not enforceable, unless your airline managers cared enough about it to try to discipline you for violating an SOP policy. My airline will not enforce it, doubt yours would either.