Also, trying to open one window opens both windows. (I added two to test this.)them!

That's probably because both windows have the "sweethome3d_hinge_1_..." in it's object source. You could edit the OBJ file and change one of the window hinges to "sweethome3d_hinge_2_..." and change the movable parts of that window to "sweethome3d_opening_on_hinge_2_...".