Fortunately there's a simple way to configure the Mixer's Output section to do this:
What you do (and you can configure this however you want) is to open an instance of the channel mixer in the Master section's FX. If you want mono from both speakers, you have to modify the preset that says 'Downmix to 1/0 (Mono)' so that what happens on the L channel (70.71% = -3dB) happens on the R channel as well. Then save a new preset called 'Downmix to 1/1 (Glorious Mono)'.
How you use this is delightfully simple - just toggle the power switch on the effect! When active, it's mono, when you turn it off it's stereo again. It works pretty much in real time, and obviously you can hack it around if you want so it's both channels to L or R, rather than both.
Incidentally, the expression 'Glorious Mono' is what the BBC has always referred to it as when it comes from more than one loudspeaker.