For Whirling Rend at least, it's clearly two separate damage rolls, because the main and off-hand weapons could have different damage dice, e.g. main hand = bastard sword and off-hand = short sword.
I agree completely, why make it so unclear and why haven't they made FAQ entry for it yet.Beyond that one easily answered question, the whole Whirling Barbarian build is one big whirling ball of uncertainty. The WotC folks have learned many valuable lessons about clarity of presentation since the introduction of 4e, but in this case they seem to have misplaced their notes.