are they in the same room?
i'm prob going to jinx myself by my DD is HSN and still takes a long nap and sleeps about 11 hours....TBH she doesn't go to bed until 830/9 these days but it is summer here..school is out and i don't mind bc she will sleep late in the morning (also really hot here so we bike ride/walk in the evening). however, my DH & I tag team it so one of us puts down a kid around the same time but they are in separate rooms.
if they are in separate rooms, i would put stan in bed and have him read by himself while you put down your DD. then i would tuck him in DD is settles. If he is capable of getting himself ready for bed (brushing teeth/PJs etc) he can do that while you are dealign with DD too.