Honestly I wouldn't suggest doing live-time replication between the apache machines. Have them have their own code, or rsync from the "main" server every so often. live-time (or near) is a lot of file inspection that just isn't necessary 99% of the time.
Personally i would recommend having three layers.
Load Balancing / Web Servers (your http/php processes)
File Servers (code/files that are needing to be shared across all web nodes)
Database Servers (your backend databases)
A lot simpler then having to do full replication between servers.
If you dont have the ability to have a dedicated file server (NFS/ect), Have "Web2" Mount "Web1's User Uploads", Both webservers will be able to read & write to the shared area, no syncing ect required unless your updating website code.
/var/www/website/_files (NFS mouned to Web1)
/var/www/website/_files.bak (rsync copy from web1 incase web1 explodes)
Both servers are near-livetime in terms of storage, they're redundant, and you dont have to add in any complex syncing nonsense.
there's a guide on how to use NFS on an EC2 instance.