On the side of using one ( or both ), you will go through the burnout, staging process cleaner and driving through the pits / return road.
For those that are concerned about damage during a backfire ... how often do your engines backfire ?
I was talked into running them front and rear in a 9375 with the 3rd circuit blocked and I didnt have any issues with it nor have the 2 people who have borrowed it and used on 3 differant engines.
I have a cup made up that fits over the rear PV on the float side and holds enough fuel to prevent it from going lean.
I also know of a N/A Altered that runs approx.1.15 60' and ran power valves front and rear. He might jump in on this thread if he reads it.