discuss.answer
An update that'll hopefully help anyone else with a similar issue. Almost 2 weeks have gone by and we haven't experienced the white screen caching issue, my last report of an issue actually seems to have been unrelated and a freak downtime incident caused caused by a server upgrade, confirmed by our hosts.
Dan, like you we came to the same conclusion that rarely but always eventually the two servers were trying to write to the same cache file. To resolve the issue, we did separate the caches as set before with a symlink - as I understand it if you use rsync in your load balanced setup you can simply exclude the cache folder from syncing. This appears to have fixed the problem for us. So if anyone does experience this issue with a load balanced set up, making sure the cache folders on each server are independent and not shared is a good place to start.
