OP
It won't cause a problem per se however Brisket shrinks tremendously and smaller ones tend to be tougher.
I would put them both in the same large pan.
https://www.tasteofhome.com/recipes/cranberry-beef-brisket/
It is fine you have slightly less pounds when making this. But like i said put both briskets in the same pan, preferably a large one.
It is best to make this a day ahead. Put in fridge cut. cold put back in the sauce and then reheat.