Is there a reason why you are not using built in movement behaviours?
This would be easily solved for example by using bullet behaviour with bounce of solids and giving the zombies solid behaviour as well..
If you want to do it your way, you might want to do a distance check, or an is overlapping, or an on collision and have one of the zombies stop moving for a while (maybe looking around dazed?) or any other behaviour you'd like..
I told my dentist I had trouble with my teeth and asked her to fix it without looking in my mouth..