Conch shells and horns

On of the most striking moments in the recent performance of Gluck's Alceste by Chelsea Opera Group was during Act 3 the Infernal God summons Alceste and is meant to be blowing into a conch shell. The composer writes for two horns, playing in unison, but the horn players hold the bells of their horns nearly together and the horns act as sort of mutes for each other. The effect is described by Berlioz in his Treatise on Orchestration. It both looks and sounds fascinating; looks, because of course, it is rather a contortion for two horn players standing side by side to place their bells together as the instruments are played laterally. And it sounds hauntingly fascinating. Though it doesn't seem to be a commonly used effect.