Load Balancing Web Servers

You shouldn’t use WP Super Cache with more than one server. It will only cache to one of the servers, so the other one may serve a non-cached page if that server wasn’t used.

You could try using NFS or SMB to share a directory but that’s not recommended. The overhead will be too much and will slow your site.

You could use a memcached based caching plugin like Batcache. That will work on a setup like yours.