They will take a bit longer all in one cooker, as you're adding 2-4x the cold meat mass that needs to recoup up to cooking heat....
You'll be fine though, if you're doing like folks said & start earlier & finish earlier & then hold....
A common scenario for me is start 9PM, run 260°, 10~10-1/2#butts,done from 9 am to 11 am,
then foil & hold
(which would cut it close for you)
So just rotate the time frame back...I just try to allow for a 15~17 hr deal, incl. some hold time
I don't have WSM's, but have been around them & read a lot....
Mine are all on kettles, set like this ~~>
8 oz. per person would be a medium amount (1 lb. raw, approx. 50% cook-down) so 25lbs. or more to start, you're way good with big 2x twins...
Hope this helps, really just confirming others...
Have a grate cook!!!