Help, can I stack 2 briskets on top of each other as my Dutch oven isn't big enough for them to be side by side?

It can be done but the upper piece will steam instead of braise, if you use more liquid the bottom piece will boil instead of braise. A turkey roasting pan with foil is a better choice...

A roasting pan or even a sheet tray covered with aluminum foil would be better choices.

I don't see why not just concerned about them cooking evenly. If braising how submerged the bottom would be and how out the top would be. Don't think I would personally do it that way but you might give it a try and let us know.