Everyone is getting too deep here. let's RTFQ and give a simple answer.... IT'S THE LIGHTER ONE.
"two jet a/c's of the same type, descending at the same speed, throttles are at idle, one significantly lighter than the other, which will have a higher rate of descent?"
In order to maintain the
same speed with idle thrust the lighter aircraft will have to increase pitch down resulting in a higher rate of descent. Those of you who have operated empty sectors in jets compared to a full load will have already experienced this many times, and some very high descent rates can be achieved in a light jet aircraft of same type at same speed as a heavy one provided thrust remains at idle.
KISS