I missed that you are using version 8.01, Please download the free update to QuarkXPress 8.12 (Windows) or 8.15 (MAC).
The link to download is:
http://downloads.quark.com/searchresults.aspx?pid=1&ftid=2&mid=2
You will get the Scale Option in this version.
To get A3 to change to two A4's you would need some work however Scale XT will make it easy to get to A4 size.
I am not sure how the page is laid out, however you may try these steps:
1. Scale single A3 page to twice the size of an A4 page.
2. Copy half the content to a new A4 page and the other have to another A4 page. off course, some elements may not fit however it will be a lot easier after scaling.
I hope I could make it clear.