That's not going to work. Firstly, that code only gives a drip-down, so it can't do checkboxes. If you want to do that you'll have to write your own code to do it. It's not that hard, you just need to think about it a little bit.
The main reason that it won't work though is that a post/page/product/etc post type can only ever have one parent, so there's no way that a user would be able to select two checkboxes and have it work as only one value will ever be used. If you want to do it this way you could possibly use radio buttons instead so that you're limited to a single selection, but I'd still advise against it because you'll fidn it gets very messy as your sites pages grow. When there's only a few pages the list isn't to long, but once you get to 20 or 30 pages there's just to many to scroll through when you display them that way.