Well, there's nothing specifically saying you can't (lack of foresight on my part, perhaps), but it kind of strikes me as not in the spirit of the challenges, along the lines of the reason why you can't enter previously made work: the entry should be something made for the challenge specifically and not be something that just happens to fit it. However, you would be making it for both rather than making it for one and entering it in the other later on. I'm also not really keen on making more rules and more limitations without a good enough reason. Personally, I'd rather see different entries just because it means I'd get to see more entries, but I suppose if Chibi Robo is fine with it then I have no reason to say no.