I don't know about jamming both those briskets on the same rack of a UDS, depending on how much of a "tight fit" it is.
You do want smoke to be able to get around the briskets entirely, so if they're jammed in there side to side the smoke's not gonna get wrap around.. and it prolly won't be too good for the rub on those sides.
Hope it all works out for you in either event, good luck!
The Life of the Knife is Ended By! The Wife.