Don't follow. Downstream means between the firewall filter and the carbs and you want to add ANOTHER one downstream?
My D has two SU pumps but they are in parallel taking fuel from each side of the tank to avoid starvation on long corners with low fuel level. But those are in parallel not in series, feeding into a Y piece and emerging as one line to the Webers.
Depending on valving and pump type, an "off" second downstream series pump could be either an impediment to the running pump trying to push fuel through it, or impeded by the dead upstream pump when "on".
I would either revert everything to stock or just buy another facet to carry as a spare.