I'm pretty sure there is no secure way to do a relogin using a iframe.
You could however install the QlikView Web Server componnet on the other server, and use it as a alternate proxy to the QlikView Server on the other domain. This is QlikView recommended way to circumvent any cross site scripting.
The procedure would be: Install QlikView Web Server on server B, then use the QMC on server A to configure it. Remember to use the same service account on both servers.
You're other option would be to create a trust between the 2 domains, which would allow SSO between them.