For glide angles of less than about 15 deg (and most transport jets glide at far less than that!), max glide range occurs at the best L/D value; this corresponds to a specific angle of attack independent of weight. If the ac has been designed sensibly, ideally it will land at a relatively high weight to yield a large payload, hence the descent IAS chosen should result in an angle of attack close to the ideal Cl/Cd max value. But if the ac is at a much lighter weight, at the same angle of attack value it would need a lower glide IAS to descend for max range at Cl/Cd max. This isn't usually possible as most ac use a fixed IAS for the descent, not AoA. So the lighter ac may well be flying at an AoA which is other than optimum; it will therefore not 'glide as well' as it will be descending at a higher rate than the heavier ac.
I did some sums on this once with simple Cl and Cd curves - and Cl/Cd all plotted against AoA. Let's say that your 'heavy' ac decends with Cl of 0.4 and Cd of 0.016 at a velocity of Vh at the AoA corresponding to best glide value. A lighter ac at 60% of the other's weight would require a Cl of 60% of 0.4, i.e. 0.24 (all other factors being the same), but from the fixed Cl/Cd curve the Cd would be 0.011, leading to a descent rate 15% higher than for its heavier brother.
The significant item in the question is the requirement for both ac to fly at the same speed, not at the same AoA!