You either lost me somewhere, or are mixing up terms, but...
Lets say domain A is your main domain (fully hosted). Set domain b to mirror it in the control panel. That part is done.
Now, whichever address you type in the address bar should stay there.
As far as page content goes, the content will be presented exactly how you have it, regardless of which domain name you type in.
You can use something like PHP to replace hard-coded references to the domain name. Example:
If you have, "Welcome to www.domainA.com!" in your page, that's what people will see, even if they go to domainb.com.
Change it to this and it will print out whichever domain you go to:
"Welcome to <?php echo $SERVER['HTTPHOST'] ?>!"
You can also keep the content in separate places, then include it based on which domain is called.
If you're only talking about how the address shows up in the browser, then you just need to go back and double-check the settings. Cloaked doesn't equal mirrored, so make sure you have it set to mirror.
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