My question is: Whay cant we do the PPB certification while doing the Deep certification? It's not like they are mutually exclusive, or I would be over tasked by doing so.
It's not about being compatible or not. You need time in the water to improve at diving and there really are no short cuts.
If you don't have the time to do both in the time you have then do the PPB. Those are core skills. Take the deep dives later when you have the time.
R..