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What you are asking for is basic frames functionality, nothing fancy at all. There are excellent tutorials on the W3Schools website - the information about frames starts here: [w3schools.com...] If you run into a specific issue as you work with developing your source code, we'd be glad to help.
What you need is a form input for the type-in area. When the form is submitted, its action attribute should be a script that changes the web location that is loaded in the other window.
I don't think it is as straightforward as it might sound, there may be problems with XSS protection.
Why would you want to do this anyway? Surely the user already has a working addressbar (complete with advanced security features)? Creating a frames browser is very 20th century. Some sites have framebusters which will ruin your effect as well.
So you'll probably need some kind of server-side script, and what kind you use will depend on what your web hosting supports. We have dedicated forums here for Perl, PHP and ASP/.NET, plus a Website Technologies Forum for the rest of the alphabet soup that might come into play - although not too likely in this case.
You could possibly do this with Javascript on the client side (yet another of our forums!) but not all your visitors will have Javascript working, and there are also the cross-site scripting (XSS) concerns that mikedee mentioned as well.