I am also struggling to understand your situation, because it seems like mine.
-I got a fully favorable decision in 11-10 for both ssi/ssdi
-then a letter from ssdi soon after stating what my backpay is/ what my lawyer got, My monthly amount, and the date it 'should' come every month. sounds right so far.
When the SSI backpay is finally calculated I know it is sent in portions, my question is; Do they not send the SSDI backpay until all the SSI portions are sent?? Because even though that seems insane, it could very well be 'par' for this course, eh?
-If you could kindly keep me posted, because my story is like your story, only one chapter behind.