Fans/pumps in series and parallel are both a done thing, at the textile factory we made the air conditioning units with 3 fans in parallel, this is not an uncommon thing to do if you want to increase output flow, in a large (industrial) chook shed it's not too unusual to see the end wall completely filled with fans, this is multiple fans in parallel, the shed itself forms a plenum chamber, the important thing to consider is allowing airflow into the plenum, any restriction will impact on the potential total flow, for e.g. a 100mm main which fed into a 100mm y that connected to each of the 100mm inlets of the 2hp DCs, that exercise would be a complete waste of time if the aim was to 2x the flow, your main has to be large enough to allow max flow into each 100mm legs of the y, further good design would be a low angle between each leg, 15 is better than 30 is better than 45 etc. and symmetry.
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1st pic fans in para, we put 3 fans in a plenum as depicted at the textile factory, note that above the dashed line (high restriction) not a lot of flow increase, could even be less flow than one fan! but keep the restriction low and potential for good flow,
3rd pic gives an indication of what happens with flow restriction,
4th is the inside view of my thicknesser outlet, this is a twin outlet and this will flow much more than a single of same size, note the symetry and a smooth transition from square to round, if (in operation) I remove a hose off one leg, air will flow into the now open outlet, this is an indication that airflow thru the machine offers more restriction than flow reversal and a sharp 180° bend, (fans fighting for air that Bob alludes to) however with both hoses in place my fan is able to produce enough pressure to overcome restriction thru the machine and the net result (of the twin hose setup) is more flow thru the machine,
4th is a 200mm main (to the left unseen) 118mm on the top side, 125 btm side, ok not symetrical but re-made for a modification for this application, 15° between each leg.

Pete