Both riders would likely have a similar (though perhaps not exact) plan.
The finishing time for each rider doesn’t really “matter” that much when creating the plans – the demands of the event will basically be the same (meaning both riders will need to train the same energy systems to prepare). The volume for each rider’s plan ultimately depends on how much time they have available to train each week, which varies per person.
As such, the plans created for each rider may indeed be quite similar. Differing Progression Levels may lead to differing intensities per workout for each rider, and time available to train may alter the total volume in each rider’s plan, but the “skeleton” of both plans would be similar.
For clarity, rider B will not necessarily get a harder plan just because they state that their event will take 8 hours to complete. Plans are built out based on what training zones we need to target for an event type and how much time an athlete has available to train each week. Finishing a gravel race in 5 hours or 8 hours doesn’t change this principle much (if at all) – the training zones that will need to be targeted are the same, and each athlete is still ultimately constrained by how much time they have available in their lives to dedicate to training.
Plans become customized from that point on thanks to Progression Levels and Plan Builder’s ability to recommend personalized volumes to each rider. While Riders A and B may be on similar-looking plans, the differences become more apparent when we look at each rider’s FTP, their Progression Levels, and how much time they are able to train per week. Those factors will lead to each athlete getting personalized workout recommendations slotted into their plans.
This is becoming a bit abstract, but hopefully it still makes sense… Let me know if you need any further clarification!