I would guess if one prokaryote engulfs another, it's highly unlikely that both of them are going to survive the experience, much less that both will thrive and

I would guess if one prokaryote engulfs another, it's highly unlikely that both of them are going to survive the experience, much less that both will thrive and evolve a symbiotic relationship, much less that this relationship will persist through cell divisions into subsequent generations. Both organisms would have to be uniquely suited to the relationship in the first place.