What you have to do is select the two halves you made (make sure it's a mesh. If it isn't, double click the subdivision object, change the "working" section to '0' and press ok. You should have the original mesh, then go "build>convert to mesh".) Now you want to select the two halves and go "build>Join solids". They're now one mesh. Then select with the drag select tool the column of points down the middle of the model that need to be merged and go "edit>Merge points" and put something small in like 0.1 since you said they're all at 0.0. The points should then be merged and you can make it a subdivision object.
