Interlinking two seperate applications

Hello Everyone,
I am working on two different web based applications installed on two seperate servers.
On both the systems, I do normal username validation and store the name in session variable.
I want to link both the systems in such a way that if a user logs in on first application, he should have direct access to the application from the second site.
can anyone help me in this plz. ?
The servers are on Linux O/S with Apache/Tomcat.
regards
rakesh

two web applications can't share any scopes (not servletContext, not httpSession).
hmmmm what if you store some information in a DB -- and when you transfer the user to the other app, you also send a key. When you look up that key in the DB, you'll see the user is currently logged in, and can grab their info from that DB?
Granted -- you'll have to find a way of doing the above securely. But that's the only reason I can think to share info between the two web apps.

Granted -- you'll have to find a way of doing the above securely.
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I use MD5 strings as cookies for that kind of thing. Two cookies, one with the user's name, then one with the MD5 of "<string><privateknownconstant>". It is more secure if the private constant is <string> dependent.
For example, say I want to pass the string "johndoe", and know for sure I passed it. I will set one cookie as "johndoe". I will set another cookie as the md5 of "123johndoes-privatekey". On the recieving side, I put "johndoe" through md5 with string value of the "private key" and see if they match. It they match, then I know I set the cookie, and the value is probably ok.
-Pete

Thanx both of you for the reply.
One thing which I have not followed is about MD5 String (Cookie ?).
As far as I understand, If I set a cokie from one W/Server, that cookie cannot be used by second W/Server. So, how do you think this problem can be solved ??
Can you elaborate more on this please ???
thanx
Rakesh

I am stretching a bit. I actually pass with a URL from one site to another although I pass between webapps in the same server using cookies like I explained.
I have not actually done the following, but is there a reason why you can't do Cookie.setDomain("other domain")? (For all I know there may be...)
Hope that helps,
-Pete