Can a male dog hump a female dog and still penetrate if they are both fixed?
Yes. Only the testicles on the male have been removed, not the penis. Penetration will still happen, as the male dog can still maintain an erection. There is just no sperm production.
Indeed, neutered male dogs can maintain an erection, penetrate a female and actually achieve orgasm, and will "tie" with the female. Most neutered males are less interested in sex due to lowered testosterone levels, but still capable of all the actions. They simply do not produce sperm and cannot impregnate a female.