Oldtimer said:No, the paladin mark and the fighter mark don't stack. Only one mark at a time on a given creature.
I can already foresee NPC defender putting mark on NPC striker to reset PC one. This is already a benefit and next turn, NPC defender can put a mark on player and NPC striker is more than happy. Same can be applied by PCs (if they will face NPC with very dangerous mark ability).
I hope you are wrong and one creature can get multiple marks. In other case, we will see some very strange tactics.