Question Can the M-One / M-One XL be operated in dual mono, without the outputs being summed in stereo

Yes, by using the "Dual Mono" Routing, you will achieve independent channels. When you select the "Dual Mono" routing option in the routing menu, you have two independent mono effects. The left input feeds engine 1, which outputs to the left output, the right input feeds engine 2 which outputs to the right output.