I want to make simple website, where on the main page, I will see login and register forms. I've been thinking about this for many hours and I'm still lost. Here is my proposed design for this:
{# Login form #}
<div>
{% render url('login_form') %}
</div>
<b>or</b>
{# Register form #}
<div>
{% render url('register_form') %}
</div>
FOS user bundle does not support such structure (login and registration in single template) out of the box, so maybe if you want to keep things simple, then you should separate both pages.
If this structure is strongly required, then you should probably extend corresponding controllers from FOS user bundle and make them use same template for output rendering.