I'm not exactly sure what's happening without doing a little experimentation. How I would solve the problem is to place an alert('value=' + document.add_product.MultipageURL.value);
as the second line of the inserthttp() because it looks to be the case that MultiPage's value is not varying from 1. If this is the case, the alert will help you figure out what you need to do to MultiPage to make it's value vary according to your needs.

cudabean