second time would be for copying the "updated luma"files to your nand , not necesary anyways, pressing just b will update luma just in your microsd ... I tried updating with x+a from 8.0 directly and it just failed, it's safe.
if you are updating luma to 8.1 from 8.0, I think it copies luma to your nand so it can boot luma without a sd/microsd, you should update pressing b first and then again pressing x+a, or else it will fail