There are no face-to face polyhedra. Silica tetrahedra do sit on the faces of octahedral openings, but the openings are vacant. There are no cations there. All oxygen atoms are shared by a tetrahedron and three octahedra. All silicon atoms are in tetrahedra.
All ferromagnesian atoms are in octahedra of two slightly different types.
Coordination Coordination polyhedra are dictated by the relative sizes of the cation and the anion. Consider a silica tetrahedron again. If we remove the silicon and insert a smaller cation, the structure may be stable, although the cation might rattle around a bit. But eventually the cation will be so tiny that it can pull three anions around itself and squeeze the fourth one out entirely. On the other hand, if we insert a bigger cation, it will push the anions apart.