Why can't two Silicon tetraoxide tetrahedrons share an edge or a face?
In solid-state chemistry, we know a certain set of rules called the Pauling rules. One of them states that sharing faces or edges is unfavorable. Imagine your silicate-tetrahedra to share faces. That would equal the closest distance between two highly charged silicon cations. They both have a charge of +4, so there is really some repulsion. Same goes for the edges which is also quite close. 

If you increase the overall size of the tetrahedron however by exchanging oxide by sulfide you will end up in the silicon(IV)sulfide structure which is essentially long chains of tetrahedra that share edges. Here by introducing a much larger anion the distance between the two silicon centers is increased and edges can be shared.