Hmm, I hadn’t thought of that! We were trying to do it with one battery. I think it would be able to support the weight of two. It’s pretty sturdy, and we found that the batteries tuck really nicely into the corner folds on the bottom one and almost support themselves (we didn’t tape them in when testing things, even though we obviously would want to for a finished project).
Thanks so much for the suggestion! We’ll definitely have to give that a try tomorrow and see if that works out better.