I think it’s possible, but it’s probably difficult. The issue is that one app needs to keep its listener socket alive while it’s in the background (hard to do on iOS) and the other app has to somehow know what port the first one is listening on so it can connect to it.
I know Android is a lot better about letting apps communicate with each other than iOS is, but I know little about Android. Maybe it’s easier over there. Maybe Apple will relent and let apps communicate more easily in iOS 8; I hope so. I get to go to WWDC this year (yay!) so I’ll be up there hoping for an announcement like that.
—Jens