This
recent thread may be of interest, as a few of the points posted are similar.
The similarity you mention between DH & DS, brings to mind Stephanie Tolan's
Is it a cheetah? Just as a cheetah needs an antelope to provide motivation to run at a good pace, a gifted kid needs an academic/intellectual challenge worthy of his/her potential to avoid underachievement. A child without sufficient academic/intellectual challenge may be academically misplaced.
The you-tube video on
Misdiagnosis and Dual Diagnoses of Gifted Children touches on myths about gifted kids, academic misplacement, and context of behavior issues, among other concepts.
You might also wish to have your child's vision tested... sometimes children underperform because they cannot see well.
