I agree with Hebe about the tag behind trailer. They are very stable and as safe as they get. I'd be worried about maneuvering something like you posted, having that much weight and bulk on the front of the bike. We had a Burley Solo. Its narrower than a 2-child carrier and there was plenty of storage room.

We just sold the Solo and bought a Weehoo for our nearly 3yo son. He LOVES riding in it. The only downside is that its not enclosed, it wouldn't be my first choice if I was commuting all the time with the kids.