Ospho essentially turns iron oxide into iron phosphate. Iron phosphate is a hard, non-porous oxide-like coating. That's why you have to cover it with a barrier coat of some sort (your epoxy or whatever paint system). You want the 24 hours for the reaction to be complete is my understanding. I wouldn't think the 48 hours would be a problem, but as others have said, always best to ask the manufacturer.
It's also my understanding that the testing is more of a CYA than anything else as the manufacturer can not test every combination of coating systems, it's just not feasible. Certainly ask both product manufacturers if they have knowledge of the combination of the two systems you are using - they may or may not have knowledge directly, but may have "close enough" data... If they can't tell you directly, then glean as much as you can and then make a more educated guess. Testing is the best method, but you do need to have the set-up to do it... That's not a small ask... One of the reasons professionals are often a good option... LOL
dj