Security :: Pass User Between Two Different Apps/websites/projects?

We have two different websites that run two seperate web sites (a backend and a front end) for our peice of software. The way we do our authentication is to put the user id into the session when the user logs in, all the pages on the site inherit from one base page if this base page cannot find this value in session it kicks the user out to the login screen.

What we want to be able to do is have a link on the front end after the user logs in and when they click on it it takes them(redirects them) to the back end. However I cannot think about how to do this in a secure and sensible way. I did think of putting the users username and password dynamically generated into the hyperlink that takes them from the front end to the back end. Would this be advisable. It will be clear text but then again we done use ssl in any case just plain forms authentication. I mean I will hash the values anyway but still be sending them. I suppose is there a way to share a session across two app domain without too much dificulty?