MultiAdd - Add a related product by checkbox
From the multiAdd perspective, it only cares that each item you're adding to the cart has some qty - if 0 it will ignore it, if anything positive, it will add that many to the cart, i.e. you only need to see things up such that for each product the minimum info required (for each product that's part of this multiadd process) is:

items[someKey][qty]

items[someKey][purchasableId]

....where of course someKey is the same value for both those inputs.

In terms of how you achieve what you want, that's more of an html form/JS question.  The most obvious approach would be to use some simple JS to listen on the submit event.  If at submit the checkbox is unchecked, you just poke a 0 into the val for that input.  If it is checked, you take the main product's qty valuie and poke that into the input.

To debug this, make sure you check your POST values after the JS is run to check you're submitting what you think you are...in the end what needs to be submitted is super simple as per the above - the only tricky part is the JS to hook it up really.