I am a Ukrainian Catholic, and our pastor was granted bi-ritual faculties, because he also served as a chaplain at a Roman Catholic hospital, and there was a need for him to offer the Roman Mass while there.
After speaking to him, I am advised that the granting of bi-ritual faculties is done pursuant to an agreement with the respective bishops, the bishop for the Roman Catholic diocese in which the priest was located and the bishop for the Ukrainian Catholic diocese in which the priest was located. The Ukrainian Catholic priest also had to undergo appropriate training to become qualified at offering the Roman Catholic Mass.
Deacons can also be bi-ritual. In the Eastern Catholic church, however, a deacon may not officiate at a wedding, because in the Eastern church, the priest is the one who confers the sacrament of matrimony, and a deacon cannot administer that sacrament.
In the Roman church, the parties are considered to confer the sacrament on each other, with the priest or deacon as a witness.
If an Eastern Catholic deacon is granted bi-ritual faculties, he is not permitted to perform wedding ceremonies in the Roman Catholic church.