If both parties are validly baptized "cradle Protestants," I don't see that there's an issue.
(Of course, if communion is offered, a Catholic guest or member of the wedding party must decline it.)
Even in “mixed marriages,” where one person is Catholic and the other is not, a dispensation to marry is routinely given by the Catholic bishops these days. Attending such weddings is perfectly okay. Even if the non-Catholic is non-Christian, attending the wedding is not a problem as long as the dispensation has been granted. Of course, it would be probable that such a wedding would not involve a Nuptial Mass (though I have seen exceptions), because a wedding between a Catholic and non-Christian is not a sacramental marriage, but is merely a “natural marriage.” Valid, yes. Just like any natural marriage between two Moslems or two Jews or a Hindu and Buddhist. But only two baptized persons can enter into a sacramental marriage. Two Catholics, two baptized Protestants, or a Catholic and baptized Protestant can enter into a sacramental marriage. No one else can, even if the Church recognizes those of other faiths as “valid” natural marriages. In the event of a person previously entering into a natural marriage and subsequently converting to the Faith, the marriage becomes sacramental at that point. People in these circumstances often have a ceremony where they renew their vows in church, but that is not the same as saying that they are only sacramentally married at that point. Those ceremonies are not a second wedding. The marriage became a Sacramental one, with all of the attendant graces at its disposal, the moment both parties became Christians.
I know most of this is off the track of your point, but I wanted to head-off a few derived scenarios some of the non-Catholics here might think about.