The "left and right outputs" are the Main L/R outputs, not aux (flex mix) outputs. Of course, you can send a stereo mix through the XLR outputs to a headphone amp and add individual aux mixes in mono to the stereo mix, which everybody is listening to. If you use Main L/R as headphone amp(s) feed, you would need to use two aux outputs for your studio monitors, which does not really make sense.
Why not use the headphone output on the front of the 24R? This is a stereo output and if you get an adapter cable, splitting stereo to a left and right signal, you can add a headphone amp. By the way, the same would be doable for studio monitors... ;-) Volume can be adjusted by the encoder and you can select the mix going to the headphone output, so this is quite a good option.