I use a setup kind of like yours, instead I have music instead of php sessions. Two machines run apache2, gnump3d, and heartbeat so that if one goes down the second will cover him. Instead of keeping the music locally and using a ton of harddrive space on both machines I use a third machine to export the NFS shares to the servers. For maximum tolerance I would like to add a fourth machine mirroring the NFS server in case it fails. Maybe this will give you some ideas.