Which part will get done faster?
In my experience dark meat gets done faster when the whole bird is cooked or when they're separated and cooked together.
Getting the two pieces to finish simultaneously is a difficult juggling act. Most turkey cooking people want the dark meat to cook to 175° to soften the connective tissue while they want to pull the white meat at considerably lower temperature, say 160°, maybe less.
I usually put the white meat in first. I put the dark meat in when the white meat IT has risen 15°. You could wait longer, maybe 25°, but it's hard to judge and your chosen ITs for pulling will vary the time interval.
I rarely hit it exact, but it's of no consequence since the fowl meat will hold nicely, even if you have to use the foil, blanket, cooler technique.