If you can't use more than 1 instance of AM security option is useless. At least you need to make authorization on web page.
The multi-user feature is mainly intended to be user with the web interface, where you can have different users logging in and only see their own miners.
You are correct that it's currently not possible to run multiple instances of Awesome Miner at the moment. That might change in the future, but will require some significant implementation changes. But it's on the long term roadmap.