Agreeing with Wyo, there´s an important difference between a male soprano (more properly boy sopranos, since there are no "castrati") whose voice is plane, with no vibrato, suggesting innocence and purity, and a female soprano, whose voice is adult, and who can suggest innocence and purity too, but can express a much wider range of emotions including drama, anger, passion, mother's love, and everything a woman can feel.
There are very few vocal works written for boys and ladies to sing simultaneously. One example could be Carl Orff's CARMINA BURANA where the children play an important but short part. Haendel's The Messiah includes a choir of children too, but not mixed with adults. Many Operas have a role for childrens choirs, neither mixed with adults.
Back to castrati (which is a pretty different thing) I personally don't think that they were in general better singers than good sopranos. Simply there were extraordinary castrati as well as there were fantastic sopranos, tenors and basses. And the good castrati were STARS of their times, deserving many master vocal works written especially for them. Let's take into consideration that a castrato voice was something very special, (as a bass woman could be) and very attractive, with all the expression of a man (because they were not effeminate at all) and the brightness of a female voice!
And back to the question, I don't see why a conductor would want ladies and boys singing the same part. Giving the part of children to women is much more common, when and only when that part was not written specifically for "white voices". Having the two voices, when needed, is the best solution, but never mixed.
Un abrazo - Andrés (not higher than E flat, please) ;)