Random "Odd" and "Even" Banners

[Mary303]

I want to have two "random" banners on each page. The code in the [default?? I am new to this, so I am unsure] template says:

PHP Code:

$random_banner[$random_number]
$random_banner[$random_number] 


Of course, this produces two instances of the same ad. Easiest way to adapt this would seem to be to have one area set to display ads with odd numbers and another area to display ads with even numbers. Is there a built-in vB function for this or do I need a mod? And if I need a mod, is there a relatively simple one that will rotate banners only on page load or page refresh? I don't need want them to rotate at any other time, don't need hit counts or banners targeted to specific forums. Thanks in advance for any help.