Hello Dan,
I also use the M-Audio FastTrack Pro, and I also have the same problem
A while ago I tried the solution you found with .asoundrc but it never worked, so I let that issue archived to fix it later as I didn't really need that functionality... until now.
I'm getting some basic recording equipment and I need to use both outs.
Have you had any luck with that solution, or have you found any other solution.
If not, let's work together to find a solution, as I consider the FastTrack a good audio interface, and the only thing missing to get all the functionality in Linux is the four outs issue.
Saludos from Colombia.
Nicanor García O.