It all depends on how your farm is setup. If you are doing some form of replication to synch files between the two, you will not be able to synchronize cache this way which means each server maintains their own cache files (just as if you used RAM). The only way this would work (and even this has problems) is if you use a fileshare for the site path (and thus the cache files) which are ready from the various web heads. Then, on top of that you would need to have the setting in the web.config for farm support setup.

Synching it every half hour does leave some room for error (ie a file not being available on one of the web heads). I know in the DNN 4.9.x version there is a problem w/ the file based caching provider, which I believe wasn't properly fixed until 4.9.4. I assume this is the provider you are using for your caching (and not the BroadCastPollingProvider - which is too much database chatter to be useable). I am not sure of your exact error, it may or may not be related to the caching bug I mentioned (can't tell without the exact error).

On another note, DNN Professional 5.1.x does offer an enhanced caching provider for web farm support you guys might be interested in.