Ed Anderson wrote:

 
 

    Hm... I still don't see the difference from flowing water that
    mysteriously won't split evenly at the Y.

Because the water wasn't splitting at a Y. It is more like this:


          ----------RAD1----------
         /                        \
---------                          ------------------
         \                        /
          ---------RAD2-----------


If the flow resistance aren't equal, one radiator will absorb flow until it's resistance equals the other.

In the case of the intakes, the air gets sucked in and then dissappears. If air didn't flow down the second tube, there'd be a vacuum.