With all due respect to “The Apologist”, I’ve never heard of any Latin Catholic bishop instructing any of his faithful that they had to attend Mass again after attending an Orthodox Liturgy, and in fact have heard several bishops say the opposite.
Citing the Canons is fine, but it has to be done in the fullness of Magisterial teaching (which does not seem to be reflected in “The Apologist’s” answer but rather a very literal interpretation of one or two canons).
This is from *Orientale Lumen *(Pope John Paul II):
Since, in fact, we believe that the venerable and ancient tradition of the Eastern Churches is an integral part of the heritage of Christ’s Church, the first need for Catholics is to be familiar with that tradition, so as to be nourished by it and to encourage the process of unity in the best way possible for each.
Our Eastern Catholic brothers and sisters are very conscious of being the living bearers of this tradition, together with our Orthodox brothers and sisters. The members of the Catholic Church of the Latin tradition must also be fully acquainted with this treasure and thus feel, with the Pope, a passionate longing that the full manifestation of the Church’s catholicity be restored to the Church and to the world, expressed not by a single tradition, and still less by one community in opposition to the other; and that we too may be granted a full taste of the divinely revealed and undivided heritage of the universal Church which is preserved and grows in the life of the Churches of the East as in those of the West.
One cannot “be familiar” if one never attends and Eastern Church. There is no negative indication here at all.
From
Unitatis Redintegratio of the Second Vatican Council, Paragraph 15:
The very rich liturgical and spiritual heritage of the Eastern Churches should be known, venerated, preserved and cherished by all. They must recognize that this is of supreme importance for the faithful preservation of the fullness of Christian tradition, and for bringing about reconciliation between Eastern and Western Christians.
Again, it can’t be known or venerated if it is never experienced, nor can the desire of the Council and the Holy Father for the understanding and veneration by Western Christians of Eastern Christianity ever occur if there is no experience of Eastern Christianity.
Another key citation is Paragraph 16 of UR (the same document referred to above):
…All this heritage of spirituality and liturgy, of discipline and theology, in its various traditions, this holy synod declares to belong to the full Catholic and apostolic character of the Church. We thank God that many Eastern children of the Catholic Church, who preserve this heritage, and wish to express it more faithfully and completely in their lives, are already living in full communion with their brethren who follow the tradition of the West.
“Full Catholic and apostolic character of the Church”. It simply does not make sense in the full light of Magisterial teaching to think you have any remaining obligation after attending an Orthodox liturgy for beneficial purposes (educational, spiritual, etc.) as a Latin Catholic.
Also, at least for Ukrainian Greek Catholics, attendance at any of the Divine Praises or the Divine Liturgy (Vespers, Matins, or the Divine Liturgy) fulfills any festal or Sunday “obligation”.