Is it okay to go out with, or have special time with, only the sibling who doesn't have a disability?

Yes—in fact, it's important. In common: in sibling support, having time when the child without a disability can have a parent to themselves is considered very good for that child's mental health. "Equal" doesn't mean treating everyone identically, but delivering what each needs. Special time isn't favoritism. What the family decides: the form and frequency can be devised. A guide: "does the sibling also get 'just me' time?" Cherishing time with each sibling one by one becomes a support for the whole family.