Glory to Jesus Christ!
Catholics are able to pray to anyone who has departed this earthly life. That they are glorified in the divine life and living as saints who can be fervent intercessors may be considered a "sure thing" if they are canonized, but that does not mean that we don't venerate others, for example Blessed Theodore Romzha. Thus, one may certainly pray to Orthodox Saints. If they hear your prayer, perhaps that can help heal the schism between the Churches?
St. John Paul II had no problem calling St. Seraphim of Sarov a saint, nor do I. Pope Francis recently named St. Gregory of Narek a Doctor of the Church, and his status as an Armenian Orthodox Christian is not clearly in communion with Rome at his departure from this life.
In terms of public devotion, the working out of veneration of saints not fully in communion with one another can become complicated.
But sometimes the most complicated things are the most blessed, so I would pray for intercession to an Orthodox Saint when fitting, and when you have the blessing of your spiritual father or mother..
