By “assist” I assume that the initial post meant to “concelibrate” or “co-celebrate” the ceremony.
This is possible in some geographical regions common to both Orthodox and Catholics as with some rites in the middle east where certain apostolic Catholic and apostolic non-Catholic groups have intermixed extended families. Regular intercommunion happens even at weekly celebrations of the Divine Liturgy and could almost be called commonplace.
They sometimes even concelebrate at times such as weddings, funerals, and other solemnities, but this usually requires the permission of both bishops in authority. I do not believe that the Russian Orthodox are open to such “concelebration/co-celebration” at this time.
A wonderful woman I was once engaged to was born into a Coptic Orthodox family. Her parents moved to Canada before her birth. She was born in Montreal at a time when there was no Coptic Orthodox Church yet established there.
When the Byzantine Orthodox were less than inviting and hospitable to them they chose this Arab Catholic Rite as their home parish. As a result she was baptized and confirmed in the Melkite Catholic Church. Later, when the Coptic Orthodox Church established a parish they returned to their home communion and she continued her worship in the home church of her family.
The Melkite Church (Syrian Greek Catholic) offers communion to all Catholics and Orthodox in good standing without prejudice. Just this past weekend I participated in one such event at the funeral memorial of a Melkite woman which included Syrian Orthodox and Melkites (as her extended family includes many members of both groups).
We should all pray that we come together as Christ would have us do.
We all could learn a great deal from this somewhat localized expression of apostolic Christian unity (I know I have) and hope it spreads all over the world.
Yours in Christ,
Veritashunter
PS - The “wonderful woman” became a nun…I told you she was wonderful. In fact, it was through her example that I was introduced to and sought out the Melkite Church of which I am now a member. God bless you, Sherin, wherever you are.