This is what I've deemed the "twin sitter". It is 1 piece with a raised edge all the way around in order to hold your little twins. :)
It is rather large but, I found it hard with my twins to put them somewhere together that gave them some moving around freedom while keeping them somewhat contained. This is perfect for tummy time (they even get to face each other going over the middle bump). It is also great when they start sitting up but still need cushion all around for those falls.
I'd love to talk to you more about this if you've got twins on the way or need it for a gift.