Setting the cookies for several sites from a single one is definitely possible, if a little controversial. The same solution has been used in the past to commit sophisticated types of affiliate fraud, which is referred to as cookie stuffing.
As with any technology, this technique can be used for good as well as evil, and from your description, it sounds like you have a legitimate purpose with this application.
It will only work with a single browser - for example, Firefox cannot set cookies for Chrome or Internet explorer.
I am sending you a PM with more details on how this will work.