I'm not sure if I've stumbled onto something or if it might actually be a real nick, but I've had a lot of success (and seen it with others once or twice) from a Seattle Slew x Cross Roads mix. It seems to be most successful when it's via Seattle Slew directly through the dam-line (as grandsire of the horse in question), through his sons Seattle Sorcerer or Northern Slew on either side, and Cross Roads, whether directly or via Backseat Driver. To find out, I've begun a little breeding project - of course, it'll take several years to prove on my own. I have a few horses to list as examples, one of which belongs to another player. I haven't looked through every single possibility in game but I've looked through a good number.
Margaritaville (Seattle Sorcerer x All Too Cross via Cross Roads) - Stakes Placed (on dirt, close 4th on turf)
Sunless Seattle (Backseat Driver via Cross Roads x Miss Tee via Seattle Slew) - Multiple Stakes Winner/Multiple Stakes Placed (on dirt and turf)
Sweet Dilemma (Cross Roads x Sweet Slew via Northern Slew) - Stakes Winner (on dirt)
Seattle Agent (3/4 sib to Margaritaville, Seattle Sorcerer x Secret Agent via Cross Roads) - Stakes Winner/Multiple Stakes Placed (dirt, turf, and jumps)
There are a few other horses out there bred on this cross, but the ones I've found have been left inactive for a year or better. One or two are unraced. I have a foal due at the end of the month from a Cross Roads mare by Golden Text, to test that route.
Southern Cross will be starting next season to test the efficacy of Double Cross in the line. Seattle Slew is further back in his pedigree, so it should be interesting. I've bought a couple of Seattle Slew and Cross Roads mares to test my theory.
Any thoughts?