Is there language that specifically prevents the mark from being overridden by another defender?
There is.
Is there language that specifically prevents the mark from being overridden by another defender?
Is there language that specifically prevents the mark from being overridden by another defender?
If not, then I don't see why a second defender can't mark the critter which will make the first mark go away. A creature can only be subject to one mark at a time...
p9 MP
3[W]+Strength modifier damage, and the target is marked until the end of the encounter or until you are knocked unconscious. No mark can supersede this one.