all we really need is another 2/3 type tbh, and stagger castle/harper enough to make it work
assuming no garbage time (big assumption given how good this team looks and shoud look going forward), you need 48 minutes with a PG on the floor. assume that castle/harper each play 32 mpg.
the very rudimentary math is they each play 8 minutes per quarter... of which 4 of those minutes are them sharing the court, and 4 apiece as the sole lead guard, ie
Castle/Harper start the game together for the first 4 minutes
Harper goes to the bench and Castle plays PG for 4 minutes
Castle subs out, Harper subs in and plays PG for 4 minutes
and you basically just mix/match these rotations for each quarter of the game, probably closing the 4th with them both sharing the floor