I never thought that you could use the same land and attribute it to both open space and habitat, but maybe you can. In the "Related Credits" section of the reference guide, the Habitat credit says that "Use of native vegetation ...may contribute to achieving the following credits:" and it lists Open Space. However, the Open Space credit does not mention Habitat in this way.
Any ideas as to whether this works according to GBCI/USGBC? Anybody submitted a project this way? Any guidance would be helpful as I've found zero information about this in CIR's, google, or LEEDuser.