The issues between the churches prevented concelebration in the past, which is essentially lack of communion at the clergy level. The laity typically could intercommune, provided they were properly prepared.
The story of the two churches has probably been covered here quite a bit, so I will not elaborate, but has to do with the complex relationship between the Russian synod of bishops in exile (and the flocks of Russian emigres they pastored) and the previously established dioceses of the Russian Metropolia in North America, which was not actually established by recent exiles but as missions from the pre-revolutionary Russian church.
The two groups have had a see-saw relationship for decades...at times closer than others. Naturally, they mixed it up quite a bit at times and for one reason or another parishes and monasteries switched affiliation.
Recent Russian immigrants sometimes joined OCA parishes, and established OCA parishes occasionally sided with the Church Abroad (ROCOR). There was a third group of Russian parishes that objected to being part of the OCA when that was granted autocephaly, preferring a closer tie to the MP. I have to wonder whether it will merge with ROCOR now.
A milestone was passed in December, to quote an OCA website: On Sunday, December 9th, Bishop Peter of Cleveland (ROCOR) celebrated Hierarchical Divine Liturgy at Ss. Peter & Paul Church (OCA) in Manville, New Jersey. This joyous and historic celebration manifested the new-found unity of the Russian Orthodox Church Abroad and the Orthodox Church in America, made possible by the re-unification of the Russian Church Abroad and the Moscow Patriarchate.
This was the first time a hierarch of the ROCOR has served the Divine Liturgy in an OCA parish since the granting of Autocephaly to the Russian Metropolia in 1970. Records might indicate that there has been no such celebration since the time of the Cleveland Sobor of 1946.
Archpriest David Straut and his parishioners of St. Elizabeth Russian Orthodox Church in nearby Rocky Hill, united with Archpriest James Parsells and his flock for this historic celebration. With the blessing of Metropolitan Herman, Bishop Peter elevated Deacon Paul Sokol of Ss. Peter & Paul Church to the rank of Protodeacon. A festal meal and fellowship was held in the Parish Center after the divine service.
His Grace, Bishop Peter expressed his hope that this celebration would be the beginning of further expressions of liturgical and canonical unity of our churches here in America. A blessed day.
Thanks be to God!
That...I think, sums it up pretty well.