Tosfos and the other Rishonim write that when the Gemara says that "neither can guard it," it does not mean that they cannot guard it at all, for then obviously it would not be a Get at all, as you pointed out. Rather, it means that they cannot guard it independently (that is, each one by him/herself cannot guard it alone, but once both of them are guarding it, it is completely guarded). Still, the Rishonim ask that it should be like a joint Reshus, in which case a Get that falls into such a Reshus is not a valid divorce. Although many Rishonim leave this question unanswered, the Ritva writes that since being able to guard the area does not actually grant possession of the Reshus to the one guarding it, and this Halachah is merely a new Takanah, there is a Safek whether the Chachamim made this Takanah in such a case as well.

D. Zupnik