With v11.5 I previously dual mined SmartCash until it was no longer profitable and had at most 1-2 ETH incorrect shares per 1000.

https://bitcointalk.org/index.php?topic=1433925.msg32298051#msg32298051

I'm very happy with the rig stability, even increasing my overclock. It's also run for 3+ days with ZERO problems on v11.5.

https://bitcointalk.org/index.php?topic=1433925.msg32771521#msg32771521

When dual mining you also need to lower your overclock.