They are about 7lbs each, so a Medium fits them comfy and snug. I say snug because you don't want them sliding all over the place but then again you don't want them cramped. They can both sit and lie comfy in the bag.
Attached are 2 of the 3 we have purchased before. The 3rd one she isn't making anymore but I will take a pic of it soon and post it.
We gave our Active-Backpack-O-Pet to our gf's when they first got their 2 yorkies. So we are going to get another one and use it for when we take the kids to the beach.