Splitting the dose plays havoc with the shed as well as with your shot schedule. If you shot at +9 (4:00) and at +15 (10:00), would your next shot be at 4:00 AM or 10:00 AM? If you wanted to shoot at 7:00 AM the next morning, you'd be shooting at PM +9 which would be a very early shot. Early shots act like a dose increase and you would be creating a large overlap with the previous shot. I think you are probably getting a sense of why this can be problematic. The easy answer is to either skip the shot unless you are home no more than 2 hours later than your usual shot time. However, if you shoot 2 hours late, you are still left with the problem of shooting 12 hours later in the morning which will, most likely, put you way off schedule.
Cindy/Mousie doesn't split doses -- she often has to shoot on an inexact 12/12 schedule due to work (e.g., she may shoot 13/11) and she shoots the same dose at each shot.