I tried it and they would not do it. However, Like Stepping said, it does not hurt to ask.
Nancy, I really think that the rules are going to really change in the future, some will improve and some will not, from a customers perspective. It really may come down to what property really wants or needs the business.
Good luck with this effort. I have tried it and got rebuffed politely by Marriott that they will not split certificates no matter how much sense this idea makes for families and others who need multiple rooms for shorter stays. This is a policy that Marriott should change so you can use an award certificate in any combination that actually provides you with a reward.