Due to the lower flow resistance through the rads, parallel rads will give you increaced flow throughout the loop, but just half the flow through each rad.
The higher flow rate through each rad in a series configuration more than compensates to the slight increace in overall flow rate with parallel rads.
The
laminar flow areas inside the rad are reduced with higher flow rates.
As for 2 in/1 out - vs - 1 in/1 out with a rez, it really isn't going to affect flow rate enough to make a difference.