Did you try copying the code that is on the top of the Q2A page? Here it is:
<form method="POST" action="./ask">
<table class="qa-form-tall-table" style="width:100%">
<tbody><tr style="vertical-align:middle;">
<td class="qa-form-tall-label" style="padding:8px; text-align:right;" width="1">
Ask a question:
</td>
<td class="qa-form-tall-data" style="padding:8px;" width="*">
<input name="title" type="text" class="qa-form-tall-text" style="width:95%;">
</td>
</tr>
</tbody></table>
<input type="hidden" name="doask1" value="1">
</form>
You would need to put that in one of your theme functions. You can check the current page using the variable $this->request.
It's possible that you put the form inside the main page for already on the page, so make sure that the two forms are completely separate.