It's not about whether the two can play together. In a vacuum, they certainly could. It's that even with that, they wouldn't draft them both in a year. So if they do it this time, it means they have no plan for rebuilding other than draft SGs until they get a star. That's ridiculously bad, especially given how they seem to view developing players nowadays.
I think a lot of people have a misconception on how BPA works. It almost never means to take the absolute best prospect all the time. It usually means making a board and sticking to it if you make a pick but to use that board to inform you when to move around the draft. If the Spurs needed a PF, but an SG was the best player on the board, they don't have to take him. They can move around until the BPA IS a PF. Also, being BPA doesn't mean to just have more talent. Positional need is certainly part of setting the board. So if Mathurin is the best player on the board, and the Spurs can't or won't find a workable trade, then so be it. But him merely having talent doesn't make him the obvious pick, especially considering that the Spurs keep finding ways to feel that way about guards while other teams to be able to find talent players at other positions just fine.