With kt & wm I explained things in an age appropriate manner. I explained that while wm could ride his bike like crazy kt had an artistic bent. In the same right, each of them had their own issues, disorders, illnesses.
As my twins have aged/matured the competition is completely intact. The difference in emotional ages are extreme; kt working at almost her physical age however has her disorders. wm is working at probably half his physical age but is doing well for an 8 year old.
I didn't dwell on the differences or the illness/disorder. I worked very hard on the positives, the talents the tweedles display. When kt is having a dissociative state & wm calls, I let wm know that kt isn't doing well. If he asks for an explanation I keep it short & sweet ~ very to the point & vice versa.
Marg has very good & insightful advice. Use it, use what works for you & yours.