How about this:
The attacker has the opportunity to make an athletic roll to transfer some of his own damage to his target, basically using the target to block their fall damage (at least partially). I don't think both should get an equal amount of damage, the total additional damage should be the usual fall damage of 5 stress per floor. The attacker can then make an athletics roll that might be opposed by a defence roll if the target can react in time. For every excess shift, the attacker can transfer half of the damage done by 1 floor to the target, rounded down, if need be. This would make it a pretty powerful attack, but still dangerous to yourself, because the most you can hope for is reducing your fall damage by half.
Example:
Joe the Vampire is jumping down a 3 story house to attack Jack, he rolls superb (+5). Jack sees the shadow appearing in the last second and tries to get out of the way, but he only rolls Good (+3). That makes 2 shifts on the attack plus (2 * 5/2) shifts from the fall, so Joe hits Jack with a total of 12 shifts. Joe himself gets the rest of the fall damage which is 10 shifts.