Use a hashing algorithm to decide to which Redis instance to go to ...
If you have three Redis hosts as example:
Then you'd typically take your key, let's say the key is appName, work out a hash for it, could be something like ((1 + 16 + 16 + 14 + 1 + 13 + 5) MOD 3) => 66 % 3 => host0
Another key could just be abcd => (1 + 2 + 3 + 4) MOD 3 => 10 % 3 => host1
As long as your algorithm to pick the server is consistent across all your applications and even if it's not, you'll simply get duplicated keys.
Alternatively go the cluster route: http://redis.io/topics/cluster-spec