Hi breezygirl, congrats on saving donnie and marie haha love what you called them.
Its normal for rabbits to get on really well in shelters then when you take them out of that environment into a new one things can change. Since both have recently been nuetered there are probably a bunch of hormones wizzing round and things are a bit unsettled.
Marie may have been aggressive to start with but her new friendly advances may still be annoying donnie because rabbits are masters at holding gruges.
I would suggest trying to bond them from scratch as if they have never met before. Are they currently living in the same cage?
I would start by having small 5 -10minute sessions definately no longer in a small area, prefferably somewhere both have never been to. See how they get on then. Immediately stop any chasing and have a glove ready to pull them apart if they scrap.
I would continue with this until they are making progress you can then lengthen their sessions.
Try and be patient it takes some bunnies ages to get over their gruges.
As for taking Donnie back to the shelter I would say that if him and Marie really are making no progress and he is continually frieghtened then it may be an idea to take him back as hard as it might be. But you have to do whats right for the bunny and what will make him the happiest.
try and stick at it though. There are great bonding tips in the bunny info sections. Also alot of the other members have gone through similar situations and they will be able to give first hand advice.
I really hope it works out for you all.