Render_to_string without render twice error

When I have a user go to a certain page, it will fork off a thread that
will perform some action. So the main thread will finish and render
:nothing => true .

Now, I want that forked off thread to render into a string a partial.
However, I cannot render_to_string :partial=> :foobar because I have
already rendered once. Is there anyway to get around this?