response.view might work better: http://web2py.com/book/default/chapter/04?search=response.view
But what I do in my user.html view is branch on request.args(0):
{{if request.args(0)=='login':}}
...
{{elif request.args(0)=='register':}}
...
{{else:}}
<div id="user_form">
{{=form}}
</div>
{{pass}}
