Validity of the marriage of Catholics of the Latin Church is determined by the Code of Canon Law for the Latin Church. If they are free to marry under the CIC and the priest of their Latin parish, where they domicile, has given appropriate permission then the wedding can take place in another particular Church sui iurus.
This means that the spouse is eastern Catholic, or the Latin Catholic has requested the ceremony in an eastern parish. If this is for a Latin and eastern Catholic, then is must be valid for both, and the canon laws are different. Some impediments to matrimony are different. Also if an eastern Catholic is one of the couple, then the eastern ceremony must be witnessed by a priest rather than by a deacon or layman. (Assuming this is not an extraordinary form ceremony.)
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