OK, so as promised, I split the rig up into NVidia and AMD seperately to see what happens. Only NVidia submitted stale shares. I even went and ran the pairs as 1 x RX 470 and 2 x GTX 1050ti, each with a hashrate of ~28MH/s.

Here is the strange one, in this setup (2 x Claymore instances, NVidia + AMD), the rig submitted less stale shares as when compared to a single Claymore instance with the cards mixed.. But it was not really stable. NVidia kept on restarting.

I am back to the mixed setup where I have a 6 - 9% stale share rate for the past 12 Hours.

Some rig info for the 28 / 28 test:
1 x RX 470, 1500 mem straps, memory OC to 2000MHz.
2 x GTX 1050Ti, +650MHz memory OC, - 100MHz CPU UC, 75% Power
Latest Windows 10 Pro, Claymore excluded from Defender
Core2Duo E8400, 4GB Ram, 16GB Virtual memory
~17% CPU Usage, 46% memory usage whilst login with TeamViewer
Polly keeping an eye on things