I believe this is a bug with the newest version of the plugin. I have an installation on my local machine with a specific page that has both the login and register forms on the page. I am working with TML 6.3.8 with WP 3.6.1 in a local environment.
I went into the database and updated the wp_postmeta to have the _tml_action request of "register" go to the same page id as the login page, and that worked, but only if the login form was displayed first on the page. When I try to display Register first it just defaults to the login form so I have two instances of the same login form.
Also, the instance doesn't actually load in the url so it will send the request to the correct instance on the page.
Finally, instances. The way I understand the documentation is that it should load "instance=1" on the first instance of the form with the instance id of 1, and the "instance=2" should load on the form with the instance id of 2. When I manually add "instance=1" into the url it will display (for instance) "Lost Password" on both forms, but if I input it as "instance=0" in the url it will display only on the first instance. If I use the "instance=2" I go to a broken form page.