Hi Nick

Catalyst has always been designed to work with a single graphics card, there has been no way to synchronise multiple cards within the Mac. I am not sure how or why you have got outputs from two cards, but it is certainly not a supported feature of Catalyst.

You can certainly get a preview of any mix/output on a preview by assigning a second, smaller mix window on the preview output to the layer(s) and as scaling within a mix is proportional, you will see what is being sent to both mixes.

Alternatively, if all mixes are keystoned mixes, on the preview monitor, setup a 'SubMix Preview' mix - this will show a preview of all mixes in one window.

I hope this helps!

Nev.