I can boot either Win8 or OS X from Solo x2 cards ... but not with both x2 cards installed at the same time (Windows won't boot). Win8 also won't boot from the Sonnet Pro card.
My system will operate normally with OS X on Sonnet Pro card (single SSD or RAID-0) and Windows8 on a Solo x2 card (Mac Pro 5,1 2012).
I would sure like to put both OS on a single PCIe slot with the Sonnet card ... but doesn't look like it is going to happen.