And I never said they'd be friendly. Certainly they labelled each other heretics and in some cases even killed each other.
But they were only schismatic because they represented a separation of the existing, legitimate powers of the church. Consider the difference between Jan Hus and Martin Luther. One ended up martyred, with his followers eventually having what, today, we would call autocephalous powers: heretical, but not, ultimately, schismatic. The other did not end up martyred, and he founded Protestantism: heretical and schismatic.
Schism without heresy is also possible.
Canon law is not a field which admits of loose definitions. Schism and heresy are different. Anybody can commit heresy. Only legitimate authorities can commit schism. Jan Hus never really got any major church authorities on his side; just some wicked good generals and good luck. Martin Luther got powers both secular and religious to come around to him (as did Anglicanism, Eastern and Oriental Orthodoxy too).
Schism is defined by its legitimate claim to some kind of authority, but rebelling against the perceived "traditional" authority (though schismatics might see themselves as "traditionalists" as often as not). Heresy is defined by its teaching of doctrines which are contrary to church dogma.
If you want another example of differences between schism and heresy, observe the many nuances of responses in Christian communities to donatism, montanism, novatianism, and, the earliest example of maybe a real knock-down drag-out fight for the heart and soul of Christianity post-Hellenizing/Latinizing of the faith: Arianism. Arianism would have been schismatic had they not been so darned confident that they were going to win the fight for authority. They, unfortunately, miscalculated the tenacity of anti-Arian leaders– as well as the price of boat-tickets across the Empire.
