I know that Canon Law only requires one Godparent, but if there are two, they must be opposite genders. What about a situation where there is only one (obviously Catholic) Godparent, but there is also a (baptized Protestant) “Christian Witness?” Can those be of the same gender, or must they be opposite genders also?
They can be of the same gender since the Christian witness has no real function.
They should be of the opposite gender. There is nothing wrong with having one Godparent. Especially if the Baptism is at a public Mass instead of being on a Saturday afternoon. This way it does not confuse the faithful in the pews.
Does Canon Law allow for such a thing? I hope not. It makes absolutely no sense to me. How can someone help raise someone in the Faith when they profess a different one?
It allows it because the Christian Witness is not charged to (has no duty, nor makes any promises to) raise the one baptized in the faith.
It should be recalled that the Christian Witness, due to their baptism, are united to the Church, but not fully. That is why if and when they join the Catholic Church, they are not re-baptized; we recognize their baptism as a valid sacrament.
They are a witness; no more and no less; they are baptized, and members of the Body of Christ, but not fully.