Have you looked into the delayed charges feature?
If the "add-on" item is the only variable that changes and the other items on the recurring invoice stay constant, you might think about creating delayed charges for your customer ahead of time so you can schedule them to send around the same time. It doesn't necessarily solve the two invoice issue, but you'd always know which items you need to charge the customer and can plan accordingly.
Thanks @vpcontroller for the video.
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