I have a both as well but only carry Hallie in them. Simon wants nothing to do with a bag
I use the sling when we go for long walks for Hallie (she doesn't walk as far) and it has one clip in it and is secure and more comfortable for me than the carrier. I like the sling because it goes across my body
I remember seeing a thread where someone else was looking for a carrier for 2. You might want to try searching for it~