These are not secondary matters. The Catholic Church recognizes the celebration of Baptism by other Christian faith communities, and a member of one of those communities who becomes a Catholic Christian is not baptized again. But he or she needs to enter into the Rite of Christian Initiation of Adults, and learn about Catholic faith, and then make a profession of faith before being confirmed and receiving Holy Communion, and so becoming a Catholic Christian. For the same reason that one cannot be baptized into two different religions, and be a member of both, it does not make sense to be receiving Holy Communion some days in the Catholic Church, while other days in another. Because of the meaning of the Holy Eucharist, it is not a true solution to the challenges of an ecumenical marriage when spouses seek to blend the religion of the husband and the religion of the wife by, for example, participating at Mass at a Catholic parish on the first and third Sunday of the month, while on the second and fourth Sundays not participating at Mass, but instead participating in the Sunday service of another religion.
Ecumenism is vital.
The Lord wills that all his disciples be one. Already, there are many ways in which Christians of different faiths can pray together, and work together in the service of Our Lord. But much prayer, work and dialogue lie ahead before real and full unity is attained, by God's grace. If our ecumenical endeavours are to be fruitful we need to recognize that the day of full unity has not yet arrived. The path to Christian unity goes by way of prayer, discipleship and dialogue.