I know exactly how you feel. When I first started volunteering at the shelter we had two senior (9 or 10 yrs old) chi's, Mario and Luigi, who were supposed to be adopted together. One of them had heart problems. They were in foster care together and were at the shelter for several months. They eventually got adopted separately as well. But I believe the foster adopted one of them.
One thing though, before they adopted them out individually they put them in separate condos to get used to being without each other.