I saw a set-up like this a while back, and never really saw the appeal, until lately, when I realized if I had a multi-spindle machine, I wouldn't neccesarily need a second machine (who am I kidding? I'd get a second machine anyway!).
Anyway, here's an example of one in action:
I have also seen one that has a slightly different set-up, with a main spindle (usually water-cooled) and a standard router as the secondary spindle. I believe the purpose of that is that you can leave a tool mounted in main spindle, and carry out simpler actions (drilling, v-carve, etc) with the secondary spindle, reducing tool changing time, etc.
Anyway, I know it's a pipe dream, but much like the modification that was made to extend the spindle out from the gantry to allow for milling pieces that are vertical at the front of the machine, sometimes someone has to push the boundaries of what they think is possible and try it. That person isn't me, since I have neither the mechanical nor technical skills to pull it off. But it might be worth pursuing if cutting the same piece at the same time, repeatedly, could save you time (and floorspace) and money.