Thanks for chiming in everyone!
1) I wonder what the riders really think? I'm sure opinion is mixed, but I know there are a number of WC pros that wish there were more races overall. This seems like a way to accomplish this (yes, its a compromise) without making teams fly all over the world more.
2) The idea riders need more recovery is kind of funny to me. Most riders do a handful of training runs, then a qualy, then maybe 1 more run, then they race. You are telling me the best riders in the world can't do ~20 laps in the entirety of a double header which spans 4-6 days?
3) It would suck to pay two entry fees, but entry fees, as pricey as they may be, are not the limiting factor for privateers. Getting to the race, in my experience, is often your biggest cost. Sure, staying in a hotel a few more nights could be pricey, too, but again, you are getting to race twice. Overall, I'd argue its more economical for everyone to do a double header.
I think the big argument against it is simply this: It sort of takes away the "purity" of WC DH racing in that its a new track every time. I've hesitated to bring up moto, but they do often revisit locations in the same season, but they build new tracks.
Maybe that'd be the best way to do it, mix the track up a little. Tape in new sections. You could use bits and pieces for one race, then other pieces for the second race.
I guess this ultimately comes down to "does everyone want more DH WC racing or not?"
I want a LOT more!!!