in all our travels I've never 'seen' any mention, warning, or requirement for any type of 'secondary' black tank protection, or sort.... that would be a ridiculous requirement in that 99% of RVs only have a single outlet and valve for dumping.
What is comes down to is simply the way the factory built and installed your plumbing and your black tank drain. Since the tank drains toward the passenger side, the exposed pipe that then takes it across to the driver side, under the exposed chassis, needs a little protection in the event something from the road hits it, cracks it, or otherwise causes it to leak or completely discharge it's contents - this is why a secondary valve is added on the passenger side. This keeps the sewer discharge from being in the exit pipe while traveling.
This is NOT typical for most other RVs since they only have valves on the DRIVER side.
So, it doesn't really matter 'which' valve you pull first, just as long as you already have your sewer hose attached and the other end connected to the outlet or dump station. Just remember to shut both when you're done, especially the passenger side one.
If you're parked for quite a while and will be dumping several times, then you can just leave the driver side one open, until ready to leave.
I know, cause our '15 Georgetown XL had a situation where this 'exposure' of the sewer pipe, that is not within a cargo bay and exposed underneath the chassis, can be struck by something in the road and cause a 'drainage' issue(!) What a day! NOT.
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
110,000+mi since '14 - US, Alaska, Canada (also, '14 Gulfstream Amerilite and '07 ForestRiver Rockwood) : )b