I experimented with the multi-add function this week and have found that if you replace type=checkbox with type=hidden on "itemnum" and set the value=0 on the RecordNumber:qnty, it works fine and you don't have to use check boxes.
<input type="hidden" name="itemnum" value="[-- PRODUCT.RecordNumber --]">
<input type="text" name="[-- PRODUCT.RecordNumber --]:qnty" value="0">
Here's a sample live page, can't wait to get the rest of my site converted.
http://www.hickoryridgedesigns.com/cj-326.html
You can add to cart to see it work, just don't check out.
Next, I'd like to find a way to select color once and have it populate all items that were added to the cart, but for now, I'm happy to get this far.
Janice