You can create a VSAN at the domain group root or in a specific domain. You can also assign the VSAN to either fabric A or fabric B, or to both fabric A and B. When you assign the VSAN to both fabrics, both of them must have different VSAN ID and FCoE VLAN ID.
https://www.cisco.com/c/en/us/td/docs/unified_computing/ucs/ucs-central/GUI-User- Guides/Storage-Mgmt/1-5/b_CiscoUCSCentral_Storage_Management_Guide_1-5/b_CiscoUCSCentral_Storage_Management_Guide_1-5_chapter_011.pdf