The Airport Express's optical output is actually buried at the bottom of the analog jack (an 1/8" mini-plug), so connecting both at once doesn't appear possible. I actually started looking for some sort of break-out cable that would let both be connected at once, but didn't find anything yet. For the cost of some of the DAC's in question, it'd unfortunately be cheaper to get a second AE to get around the problem of the combined jacks.

It might be interesting to compare the analog and digital outputs when used with the main zone - MP3 is a medium that's already been a bit "squeezed" sonically so the benefits of a better DAC may not pay off as much as with other sources, and using the analog all the time might be an option.