I have a 14lb brisket and a 10 lb,brother wants to smoke them this weekend at same time,how,and approx how long?

I always place the larger item closer to the heating element (on the bottom). I have not seen a linear realtionship between mass and time...for example, 2 10 lb. cuts do not require twice as much time as 1-10 lb. cut. The cook times in my 055 increase by 10-15% (just guessing here as my logs are not with me now) when it is loaded vs. a single cut.

I've cooked multiple briskets before without a problem. It isn't any different from cooking one. Just probe both--you might find the one on top gets more heat.

Place the bottom one fat side down; the top fat side up. Then flip and rotate shelfs part way through.