Here is the basic idea that I could glean quickly, should put you on the right track:
Template: /ideal_responsive/modules/One_Page_Checkout/opc_summary.tpl
Code:
<div class="terms_n_conditions">
<label for="accept_terms">
<input type="checkbox" name="accept_terms" id="accept_terms" value="Y" />
{$lng.txt_terms_and_conditions_note|substitute:"terms_url":"`$xcart_web_dir`/pages.php?alias=conditions":"privacy_url":"`$xcart_web_dir`/pages.php?alias=business"}
</label>
</div>
Just duplicate that code and change the input name to "accept_two" or similar, update the label "for" too. Wipe out the language variable in the label and put your own text and links. Then search for the JavaScript and update the function to check for the second box, and that's the piece that would take me a bit to look up and work out for you.