Would have to be per giver, otherwise it would be trivial to avoid (want Dave to get another $1m, either give $2m and have the taxes taken, or give $1.1m to Joe

Would have to be per giver, otherwise it would be trivial to avoid (want Dave to get another $1m, either give $2m and have the taxes taken, or give $1.1m to Joe Bloggs, who gives $1m to Dave and keeps the rest.