Anyone know if you can buy the W6800X MPX Module and add another 6800 card later, i.e,, make it a Duo later?
Also, why isn't the 6900 available as a Duo?
You can't convert a W6800X to a W6800X Duo but you can have two separate GPU's in one MacPro system. Two separate GPU's will probably perform slightly better judging by Apples specs.
The W6900X is probably too hot or power-hungry (or both) to successfully work in a duo-config. Apple has even down-clocked the W6800X Duo's a bit, likely due to the same issue. As with the W6800X you can however run two W6900X in a MacPro system. This will likely be the most performant solution in an app like Resolve but cost vs speed is off the charts.