There are a couple of ways this could be done, depending on the filter code on the products_results.php page. Probably something like:
https://www.thestonecenter.com/products_results.php?Category_ID=34,35&Search=1
or:
https://www.thestonecenter.com/products_results.php?Category_ID[]=34&Category_ID[]=35&Search=1