One force must stop their movement if enter in one enemy fort or fortress with any siege marker (logically with anothers troops making the siege)?
Rule 6.51 seems to indicate that your force does not have to stop if it enters a space where you are besieging the enemy:
6.51 A unit or force using any kind of movement to enter a
space occupied by unbesieged enemy units or fortifications must
stop and take the appropriate action as listed below:
I found nothing else that would help answer the question...