Posted for "Bill&Sue" <5zq@cox.net>:

 Terrence,  forgive me for my poor explanation.  Let me try again.
 
 The fuel lines from the wing tanks both flow to the crossflow valve. Think of
a line running along the cockpit floor connecting the left wing tank to the
right. Now put a valve right in the middle to allow or prevent the fuel in the
wings from flowing back and forth (by gravity if one fuel level is higher than
the other). Then, on either side of this "crossflow" valve insert a tee. Each
tee connects to it's respective Facet pump. The output from each Facet pump
flows to the header tank via it's own dedicated line. Hence, the two lines to
the header.
 
 With the crossflow valve open, gravity keeps fuel available to either pump
untill both tanks are completely empty. It can't suck air until there's no
more fuel left. Even if it did (prolonged slip?). The engine is being fed by
the header so theres no problem there, and the wings will equalize as soon as
the slip is abandoned.
 
 Bill Harrelson
 N5ZQ 320 1,175 hrs
 N6ZQ  IV  6.0012%