If you run more than one ads, but lack of space to put the ads, you can actually rotate the ads.
The code below will create a random number from 1 to 10. Since I want both ads to be shown with a probability of 50%, I put the condition if (x > 5). However, you can change this number.
<?php $x = rand(1,10); ?>
<?php if ($x > 5) { ?>
<!--your first ad code-->
<?php } else {?>
<!--your second ad code-->
<?php } ?>
You can read more about PHP rand() function here.
<?php if ($x > 5) { ?>
<!--your first ad code-->
<?php } else {?>
<!--your second ad code-->
<?php } ?>