I am cooking a Brisket. I have 2 briskets 1 is 2.3 lbs and 1 is 2.0 lbs I have to cook them in the same oven. How long should I cook them for?

If it's not too late to help -
most versions I know cook the brisket between 350 and 400F for about 3-4 hours for 4 to 6 lb of meat. And/or to internal temp of 175F.
And the (classic, much heralded) Nach Waxman recipe cooks about 6 lb meat 3-4 hours at 350F (recipe many places, including food52).
So use those times and heat ranges, plus a meat thermometer, for cooking your total 4.3 lb meet.
Maybe check them at 1 or 1 1/2 hours in case the smaller cuts are cooking faster.