Not on the road particularly. I've done it in the driveway to see what it did and the other day I tried it Progressive and that's about as close as I got I guess..
The reason I ask is it's demand of CFM's that pulls the back flaps open. You may not be drawing enough to pull both doors open, so both stay shut. IF you run off of just one carb, and it pulls that back flap open in a second, then that answers it.