I would order 1/2" over 3/8" considering it's nearly the same cost overall and you will have double the blocks to restrict flow.
When you order your 1/2" Maze 2 blocks also specify "upgrade" in the commemts area so you will be sure to get the new version, which cools about the same, but flows much better, which will benefit you with the dual setup
As for pumps I think a Mag-Drive 350 will probably suffice. I have a single Maze 2 1/2" "upgrade" and it nearly doesn't slow the flow down, so I'd like to say the second one couldn't do much harm either. Even the 350gph version places a lot of heat in the system, and as the ratings go higher there wattage seems to go up exponentialy.
Lastly for rads I'd definetly do something with lots of headroom like a Big Momma, or D-Tek Cooler Core. I'm swayed towards the D-Tek now (even though there new) because they have 1/2" fittings and are only $42.00. A single high quality rad will work better than two (proven in motorcycle racing as the differance in air to water temp is lower in the second rad, lowering cooling efficiency).
Your one of the first to try the dual athlon setup, so I can only speculate this rad will be enough. As for routing, I'd go inline (without a resevoir, just a t-fitting), and run in series between everything. Your flow should be high enough and the difference between cpu temps in intel situations is usually only a degree or two. Sometimes if you go parellel the flow (and cooling ability) drops and causes both blocks to be even higher, but hey like I said, you'll need to experiment (it'll work either way, and tubings cheap, watercooling is all about experimenting)
Please definetely follow up results when you're done, I'm sure many of us will be very interested!!!
