Well, that parallel world is entirely inaccessible to us. It wouldn't be inaccurate to say that anti-ado doesn't exist in the cosmere- maybe it exists in the anti-universe, but we have no hints of parallel worlds in the cosmere, so that's just random theorizing.
There can't be any anti-shard or anti-ado in the cosmere right now, any more than there's an anti-planet or anti-star. It would all be annihilated instantly in the spiritual realm. If it were in the physical realm, well, that has its own issues, and if it were in the cognitive, a mix of the two issues would have appeared by now.