Lynea, who previously played Anna Of Cleves, and composer Marlow performed in the show as sickness struck down other members of the cast.
Joined by Jarneia Richard-Noel, Millie O'Connell Courtney Stapleton and Aimie Atkinson, they staged a special concert version of the show.
You can watch Toby Marlow and Genesis Lynea in Six The Musical's Megasix from Sunday, July 28 in the video below.
In a statement, Six The Musical said on social media: "Queens! For those attending the 7pm performance of SIX today 👑 Due to illness and injury which has stricken the cast over the past few days we will be performing a unique concert version of the show this evening.
"For those of you familiar with the album...Genesis Lynea, the voice of ANNA OF CLEVES on the album will be singing that role this evening.
"And...drum roll please Maria...TOBY MARLOW who wrote SIX will be performing the role of CATHERINE PARR. This really is a first and enables us to give you a performance today.
"We are sure that you are going to have a wonderful time at this evenings performance, but If you would like to re-book to see the fully choreographed show another time, please keep your proof of purchase and contact your ticket provider.
"We will be offering complimentary tickets for another performance of your choice in the future. Thank you for your support "
Nominated for five Olivier Awards, SIX turns Tudor queens into pop princesses as 500 years of historical heartbreak becomes an hour of 21st century sass.
The show - written and composed by Toby Marlow and Lucy Moss - has become a huge hit since first premiering at the Edinburgh Fringe in 2017.
Six The Musical is currently playing an extended run in the West End at the Arts Theatre, with performances currently booking until July 2020.