I like this pen so much I bought two. I don't know why other posts say you can't combine two pens together. [I DO.]
I've never had a problem with them falling over. (I do have 2 small dogs) Boston Terrier & Minature Pinscher. Maybe with larger dogs they would be as stable hooked together, but they work fine for my use. I've used them on sand, gravel and concert. Never had an issue. Having 2 gates is a plus too!