I don't have time to code but this is not a PHP problem, this thread should be moved to JavaScript.
Assuming you know the total number of checkboxes (fixed number), what you would do is in each column you would have a <div> container for each checkbox.
Of course in the beginning, all the checkboxes are in the left hand side and the right column <divs> will be empty.
then using the innerHTML() value, you move the content from the left div to the right one, such as.
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rightDiv_1.innherHTML = leftDiv_1.innerHTML; // copy it over to the right.
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leftDiv_1.innerHTML = ""; // empty it.
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put the above code in a function and call it with the checkboxe's event trigger (such as onClick())
I hope that gives you an idea of where to start. If any problem, post it in the JavaScript forum. I'm sure those guys are much better JS scripters than me =)
Thanks,
Dan