Its called a double cross. Check it out...
http://passel.unl.edu/pages/informa...rmationmodule=1099683867&topicorder=5&maxto=6
Mathematically the numbers are similar as an F2, perhaps more phenos.
So according to that article, there's a better chance to find a "special" plant by just doing the initial f1 x's of each pair. Run seeds from each "lot" instead of taking the next step and x'ing the f1's together.