[pdf27]

Looks about right to me.
For the block P/Q curve you should be getting total flow rate equals the sum of the flow rates for the two blocks at whatever dP value you're using.
From this, you can set the operating point of the pump (point where the two P/Q curves intersect). This then gives you the actual dP value for the blocks (after allowing for radiator, tubing effects etc.), which in turn gives you the flow rate through each block.
You presumably can already calculate the flow rate in series (just add the dPs rather than the flows), giving you the block flow rates in series and parallell for this pump. Getting performance figures from pH's testing is dead easy at that point. It's also probably worth doing as there are a number of hardcore silencers about who would accept a fairly minor temp rise in exchange for a marginally quieter pump (which it might be if you're no longer operating at one end of the flow curve).

Just be careful with your "one restrictive block" analogy - you may well end up running two rads in parallell, which is unlikely to model most situations accurately.