Because they are not heretic, and shared communion for ~ 1000 years.
The split between Eastern Orthodox and Roman Catholicism that we take for granted now was crystalized in 1054 when the leaders of both of those Christian communities pronounced anathema on each other. (This is also described as a mutual excommunication). Before then, the Church of Rome and the Church of Constantinople were in communion with each other, and had been since The Church had begun, with a few notable and public "splits" during the Iconoclasm controversey and a few other points along the way (such as the Henoticon incident).
There is no heresy: you got your terms wrong
Or maybe it means that there's no such thing as heresy.
Given that The Church (before the Schism) spent a great deal of time fighting heresy, and from that fight came the Nicene Creed that the Catholics and Orthodox share, and after that the Council of Chalcedon wherein communion was lost with the Coptic church, no, that is a bad guess.
And as far as terminology goes, schism and heresy are not the same thing. That is explained in detail at the linked Q&A.
The veneration and acknowledgement of the Saints was already a long standing, centuries long tradition and shared belief among Christians before 1054 AD when the formal split (The Great Schism) occurred. That means that both faith communities carried forth their veneration of the Saints into their (for the time, separate) futures.
Many saints were already recognized long before the Great Schism
Saints that were canonized or acknowledged by the Church before the Schism are venerated by both Churches to this day. For saints canonized after the split (the process has evolved somewhat over the centuries, there's a detailed article here) I'd suggest you check on a case-by-case basis for a given saint. For example, St Vladimir the Great, and St Seraphim of Sarov (called Saint by Pope John Paul II in his book Crossing the Threshold of Hope.
Saint Paul, Saint Peter, the Apostles, Saint Gregory? In long before the schism.
Heretic is a false characterization
As a result of the process of dialogue during Vatican II (1962-1965) the mutual pronouncements of anathema (the mutual excommunications) were rescinded. The two churches still view each other as either estranged or schismatic to one extent or another, since some fundamental points of faith and teaching are not agreed. For example, the "Filioque" disagreement remains unresolved. (The answer to that link by @AthanasiusofAlex is well worth the read).
There's decent coverage of the relationships between the two churches here.
Significantly, at the close of the council Pope Paul VI and Orthodox Patriarch Athenagoras mutually lifted their respective excommunications in the Catholic–Orthodox Joint Declaration of 1965. This, however, was largely symbolic of intentions to eventually restore full communion between the churches.
That last objective has not yet been achieved. Whether it ever will be remains an open question, and among many Christians a hope for a united future. There is hope for reconciliation.
How can they both have saints?
1. They share a lot of saints in common based on the pre Schism saint assemblage.
2. Each Church, since the Great Schism, holds themselves as "the true church" and thus each has standing to canonize a saint by their own criteria. Whether or not they agree on any saints after the Great Schism is irrelevant; where the faith community identifies someone as being a Saint, they declare such and it becomes a part of the sacred tradition. There is a related Q&A here.