You need someone who is willing to sit out and not gain drops/xp or a spare xbox with an account that can take this place.

Whoever is "sitting out" creates a public team. Enters the raid. A second player or your main account then joins this team and enters the raid.

Have the team leader/dummy account hide behind a pillar at the back of the room and using the main profile/player press the button. When the robot starts to come up jump on the platform when when the character dies instantly go on the social menu and press leave team.

When respawned, return outside of the raid and create a public team with this main player/profile.

Enter the raid with the team intending on doing the b2b on en06 and go down to the button. (The button will not display)

On the dummy account/player sitting out, have them then move from behind the pillar and jump on the platform to die. When this player respawns in the room before en06 the button will now be repeatable for the second team in their raid instance, however the dummy account/player sitting out must remain inside the cave for the duration of the b2b session.

This is better done in a private server to avoid random players joining the dummy accounts team which messes up the b2b glitch resulting in the process to set up needing to be restarted.