I think you should def. get him neutered asap. At 9 months, those teeth most likely aren't coming out... I would just go ahead and get it over with. Does Carmel mark indoors? Jackson got neutered at 7 months old and he will still mark outdoors (which I don't mind...) but never has indoors. I would also neuter the 6 month old as soon as you get him.... two neutered males, in my opinion, will get along much better and be happier, healthier pets.
But to answer your question, even after getting neutered, Carmel will probably still mark outdoors but it won't be as excessive.