2
As you have discovered, a Smarty variable points to the same value no matter how many times you place it in your theme.
You will have to edit the xoops_getbanner() function in include/functions.php to return an array of banners which you will then be able to use with <{$xoops_banner.0}>, <{$xoops_banner.1}> etc. depending on how you build the array in the function.