Janke has explained the way SS calculates concurrent benefits in-depth multiple times and some of the finer points are still a bit confusing to me, so I'll provide a link:
Social Security - The Red Book - Concurrent Benefits and Employment Supports
You should be aware that your SSI backpay will be determined at your local office first and the SSDI backpay is done after, so it will take longer. If something a like a WC offset is also a factor, expect additional delays.