[tbworld]

Off the top of my head, you going to need the $perpage variable on showthread, which is already registered to the template. Obviously, you will need to divide ($threadinfo[resultcount] / $perpage) and handle it correctly, but I have not looked at the code as of yet. When I get a chance I will do so.

// Hook: postbit_display_complete
//   Settings->Options->Thread Display Options (showthread)->Maximum Displayed Posts Before Page Split.
//
// Use the following variable for your condition in your selected postbit template.
//   The following variable contains an on-going post count from the page query.
//     {vb:raw tb_postbit_postcount}

if (THIS_SCRIPT == 'showthread')
{
    global $tb_postbit_postcount;
    vB_Template::preRegister('postbit_legacy',array('tb_postbit_postcount' => ++$tb_postbit_postcount));        
}

You will need to do some tests on this, but it should work. Now you can place an ad on post two and on post six, per page. Have fun.