Here's my $0.02:
Before I get started, let me just say that pressure is not the same everywhere in a closed loop (Bernoulli will tell you that... even if the water is not moving the pressure is not the same).
Ok then. I like both of Bigben's suggestions concerning the pump setup. I don't think that one can be recommended over the other however, since the two pumps are different (and as such, have different Q-H curves). As far as I know, there are two ways to determine which is better in this system. First, you could simply try both, and see what happens. Otherwise, you can calculate a resistance curve for your setup for various flow rates, then create a combined pump curve for either series or parallel operation, and find out the operating point for both setups (not that I think there's enough information available to do this... when I asked DD what the pressure drop through their Maze 3 was all I got was ignored
). The only suggestion I have is to try the rads in parallel rather than in series, since I bet they're more efficient that way.