eltano343, normally you should have some kind of common storage that is shared between all the instances if you want that behavior. NFS technically works, but it's not very fast.
Having a big NAS is better but probably not an option with AWS. In the end, WPSC is not really built for that type of environment.
You may want to look into using a Memcached server with Batcache or W3TC. WPSC also as a Memcached option I think, although I'm not sure how solid it actually is... Not a fun choice, but having a shared storage (in RAM or disk) is the best way to scale.