Two Subwoofers Connection Advice

I don't think this applies in your case.  I think Velodyne is suggestion you create a monophonic sub signal.  that is, each sub combines L and R, hopefully improving room response.  It's not a bad idea.

In your case however, sinc the Marantz has dual sub outputs you can forego this and deal wiht it all in your bass management menu. 

Does the Marantz  label them sub 1 and sub 2 or left and right?  Can they be assigned Left and Right in the Marantz menu ?   That would be ideal, left to left and right to right.  

You could Google your module with that question if you don't have the manual.thats what I do and it's seems to find the answered on my denon and marantz avr.there getting alot better with the newer av 10 4 subs out with dsp for each in the room.enjoy the education and the music