It's possible the explanation given in "The Sinner's Grand Tour" is incomplete for some reason ... ? I can imagine a somewhat conservative person limiting it to a twosome, deeming the notion that a third party was laid on that lower cushion, licking and probing the jiggling and thrusting royal undercarriage, may not have been considered an acceptable revellation for upstanding morality. ; )
I actually have the book and both Perrotet and the chair's current owner (scion of its original builder) say Bertie used it with two ladies. Sadly they provide few details on the mechanics. I'll dig up the relevant passage.
__________________ Old Pain In The Ass says: I am on a mission from God to comfort the afflicted and afflict the comfortable; to bring faith to the doubtful and doubt to the faithful.