How do you switch a 557 into Mono mode?

It has a switch to switch it into bridged mode, but then the only way to attach two pairs of loudspeakers that way would be in parallel. This would take a 4 Ohm speaker to a 2 ohm load and strain the amp a bit, though it would probably work with a beast like the 557 bridged.

Office: Meridian 500.1, 568.2mm, 557, 505, Dali Helicon 800 L/R and C200 Mk2 Centre, BK Monolith FF cool
Cinema: Valerion Visionmaster Pro 2, Arcam AVR31, KEF LS50 Meta LR, T300c, Focal Dome On-Walls, PMC Twenty.Sub
Bedrooms: Meridian 2x M80