Although it is an awesome feature, there are not many sleeping bags out there that can be zipped together to make an even larger sleeping bag. Largely, this is due to the fact that the wide majority of sleeping bags on the market are mummy style sleeping bags, which pride themselves on incredible warmth retention due to a more refined shape.
However, envelope style sleeping bags are becoming more and more popular. While these rectangular bags cannot compete with mummy bags when it comes to warmth retention, they are particularly well suited to summer applications. They are usually thinner than mummy bags, but the zipper extends all the way around the bag, allowing you to fold it out and use it as a blanket, or, like the question suggests, zip more than one of the same style sleeping bag together to make one big two person sleeping bag.