One of personal pet hcsI have is that if Doctor isn't just a very talented and very fucked up collegue of Priestess she grew close to over shared dreams and visions.

Then the person we know as Doctor appeared when Priestess became Priestess. Literally or metaphorically. It does apply even to general "Priesstess and Doctor are just very good friends" idea but. idk, I think it is vital that you understand the alternative I specifically envision in my head as. What is knowledge. Can you truly define the Universe. What does it mean to exist. You categorize, you use your perception, the cognitive abilities your species developed over millenia. You think and ponder and engage in theorycrafting, and one of the results of this thought, of this attempt to tackle what, perhaps, may for all we know be impossible to define. As science. If Priestess ever ventured beyond the wall that we call "the known", then what she brought back from here she must have also coated in the definitions that can be known. The science. The doctor. Her closest most intimate friend. That's the alternative very dubious hc I propose.

In between we can posit many more possibilities of various levels of "two researchers really do some dubious things together and to each other" of course.