There's already been a few "rehearsals" of the three way splits and joins in Waverley. It's clearly not feasible loco, crew and probably paths wise to do a full dress rehearsal, but not sure why that's needed as long as the individual routes and the Waverley shunts have been all tested a few times individually - the only difference in doing the three parts + shunts together is making sure they all arrive at the right times in Waverley, but that's 'practised' that every night...
Good job they have AAR multi and can work with 66s and 67s