"but my experience has been that you can have two different briskets at the same weight and they can still cook drastically different even under the same conditions."
This is very true.......tough old toad of a piece of meat & it's hard to tell "when" even sometimes for experienced people.....
If you'd like to do them all on one, I'd set on the smaller one 3~4 hours later & you MAY come close to even doneness around the same time.......only downside of doing it that way I can think of is it might be harder to manage / add coals, depending on your setup........
Two different kettles is maybe easier, depending on how you work......I'm reluctant to TELL someone exactly what to do in a lot of cases, 'cause they may be perfectly comfortable with the "other" way of doing things......hope that makes sense.......^^^^^^^^
normally, I would run 240~260f.....moves a little faster w/o the risk of going wild & drying out........