My daughter is a DAC currently receiving $640 in SSDI on her own record. Using Max my SS, I will retire at FRA in October, 2018 and receive PIA of $2728. FMB is $4,776. SSA is telling me that my daughter's SSDI will terminate and she'll be entitled to 50% of her share of the FMB (4,776-2728=2048x50%=1024). My wife gets the other 50%. My understanding was that she would continue to get her own SSDI and receive the difference between 1024 and 640. Also, since she is dual eligible, can she claim the full 1024 and still get benefits on her own account?
Let me preface by explaining that although I answer the general Social Security questions submitted to this forum, I don't have access to the data entered by our software subscribers nor their results. Specific questions regarding the software or your results should be submitted via an online contact form in the help menu.
Regarding your question, your daughter would not get both her own Social Security disability (SSDI) benefit and her full share of your family maximum. Instead, if your daughter qualifies for Disabled Adult Child's (DAC) benefits she'll continue to receive her own SSDI plus the difference between that amount and her normal DAC benefit rate. So, assuming that your figures are correct (i.e. PIA = $2728, Family Maximum Benefit (FMB) = $4776) and if your wife qualifies for spousal benefits on your record, your daughter's DAC payment amount would be $384 (i.e. ($4776-$2728)/2 - $640). Thus, her combined benefit rate would be $1024 (i.e. $640 + $384).
The fact that your daughter would be dually entitled (i.e. entitled to 2 different benefits) would not cause her to receive a higher overall benefit rate. However, it could cause your wife to receive a higher spousal amount if she's not also dually entitled. Since a portion of your daughter's share of the FMB will not be paid to her because of her SSDI, that amount can potentially be redistributed to other non-dually entitled beneficiaries on your record. That could potentially allow your wife to receive up to her full spousal rate of 50% of your PIA, or $1364 (https://secure.ssa.gov/apps10/poms.nsf/lnx/0300615768).