Thanks to everyone for the feedback! You certainly brought to light a few things I hadn't thought of (sometimes, I get so focused on the pressure and flow ratings, I forget about things like chatter).
So maybe explaining the system would help. Then again, maybe it'll just be more confusing...
I have 2 PSVs that protect the same vessel. One is 2J3, the other is 6R8. The vessel in question has a MAWP of 75 psig and both valves are set at 75 psig. We are running up against the relief flow rate (calculated using API 520 and 521 at 116% overpressure) and the lift pressure of the valves...