Damage taken to mana vs damage gained to mana.

You gain the mana before it's subtracted from your life (or in this case, from your mana). You'd end up with 0 mana after your theoretical hit, but a second hit would then gain your mana up to full before it was taken down to 0 again. So you'd effectively still be "absorbing" half the damage, but your mana would be empty, not full, between hits.