Okay, looking at the various radiators from BillA's tests, and the finnage and surface area, and a bit of hand-waving, and I'd roughly project that the two Camry cores in parallel would have an air-flow pressure drop of around 0.05inH2O at 100CFM.
Munching through the
PQ curves for the fans (at 12V) it would appear that a ~200CFM total air-flow would be predicted for the two fans in parallel (~0.25inH2O pressure drop matched off against each fan's PQ curve).
Since 200CFM is about what even a single fan can push in free-flow mode, the prediction would be that in this scenario that the two fans in parallel would be the better option than in series.
If the fans and radiators are split into their own separate plenum chambers, the problem becomes a simpler one of predicting air-flow for a single fan against a 0.10inH2O at 100CFM scenario. Here we can roughly determine that each fan would push ~140CFM through a single radiator, so clearly the separate plenum chamber operation is much better, since we have now boosted per-radiator flow from 100CFM to 140CFM just by separating the two parallel fans acting on two parallel radiators into a pairs of single fans acting on single radiators.
In series the total air-flow works out to around 180CFM.
So I can somewhat predict that for my particular scenario with fairly large free-flowing radiators, that a separated pairing is better than a parallel pairing, which is again better than in series.
For a more air-flow restrictive radiator setup (say if I were only using a single radiator), then the in-series option would work out better.
Time to do some more wood-work...