if you run two pumps in parallel you need to use check valves inline in case of one pump failing. ON PAPER, two pumps in series will double the pressure ...but in reality they wont. Two pumps in parallel ON PAPER will double the availiable volume.
Any electric fuel pump, (or mechanical for that matter) especially with carbs such as Webers (down or side drafts) SU's and any carb that wants to see low pressure, NEEDS a pressure regulator. A decent regulator. Not the regulator that Purolator markets with the adjusable wheel. Were dealing with small increments of a pound in these carburetors and not + - a pound or so. and you need to set that pressure with a reliable gage and not "by ear".