§1. The prescripts of can. 1108 are to be observed for the form to be used in a mixed marriage. Nevertheless, if a Catholic party contracts marriage with a non-Catholic party of an Eastern rite, the canonical form of the celebration must be observed for liceity only; for validity, however, the presence of a sacred minister is required and the other requirements of law are to be observed.
§2. If grave diYculties hinder the observance of canonical form, the local ordinary of the Catholic party has the right of dispensing from the form in individual cases, after having consulted the ordinary of the place in which the marriage is celebrated and with some public form of celebration for validity. It is for the conference of bishops to establish norms by which the aforementioned dispensation is to be granted in a uniform manner.
§3. It is forbidden to have another religious celebration of the same marriage to give or renew matrimonial consent before or after the canonical celebration according to the norm of §1. Likewise, there is not to be a religious celebration in which the Catholic who is assisting and a non-Catholic minister together, using their own rites, ask for the consent of the parties.
Source: Code of Canon Law (Vatican.va)