If the box is mono, what's the point of creating a dual mono feed? It will still sound mono and will not stereoize the signal in any way. If you are recording to software, it would be much easier to record to a mono track, it will output to both left and right channels anyway. If you are feeding the signal to a mixer, plain center panning usually works.
Btw, if you have any pedal with a stereo output, why not make that last in the chain? AFAIK, stereo delay pedals usually output to left and right. The hard part is finding a stereo DI box. You can also route to a digital multifx which usually output to stereo line outs anyway.
