Hi there!
Unfortunately TwinAx is a giant mess, and we don't have too many options to fix it.
We only certify Cisco and NetApp branded TwinAx cables, and I know from personal experience that the VDX6740 does not support either of those.
While the Brocade TwinAx cables will work in many NetApp 10Gb adapters, it is not supported and is flagged as such.
This means - the supported option is to use optical.
Unsupported options include the twinax (DAC) cables with a different vendor ID on each end, otherwise known as dual coded DAC. Usually works fine, but not officially supported.