Most desktop class boards can only support bifurcating a 16x lane slot into two 8x lane slots, so it's more about preventing user error, as 2 M.2 slots would li

Most desktop class boards can only support bifurcating a 16x lane slot into two 8x lane slots, so it's more about preventing user error, as 2 M.2 slots would likely not function in most cases. Even on server class hardware, the split options for a single slot are usually (but not always) 16x, 2* 8x, and 4* 4x - an 8x + 2* 4x option is somewhat unusual.