Hey cal, I think this refers to hooking up 2 cabs in both 4 ohm jacks of A or B, like hooking up 4 8ohm cabs in all the 4 ohm jacks and then using the A+B setting.
You could give Boogie a call and verify this, but there shouldn't be any problem if you're just hooking up 1 cab in A and 1 cab in B.