How exciting! I think getting siblings is such a great idea -- they will always have each other. Transition will be so much easier and you will have double the loving!
Celtei has a fantastic double carrier that would be a good investment if you are going to use it a lot. They are nice quality and they do sometimes have sale and special deals on their website.
Edit to add the website:
http://store.celltei.com/ check on the custom carriers page and it will slide show through several variations, one of them is the double carrier. Several other styles can be purchased in a size convenient for two also.