You need a separate form for each button for "_xclick"
You should be able to get it working with "_cart".
I think you also have to add:
<input name="upload" value="1" type="hidden"/>
to add multiple items with an "_cart" command.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.