I'd imagine they'd have encountered one another by now. I suppose divergent evolution and the general bad temperament of the snappers would set them at odds with their landbound kin.
Since they both inhabit the Savage Coast in Mystara they can hardly not be aware of each other. Since Snappers prefer the ocean and Tortles prefer forests and beaches they presumably mainly encounter one another along the shoreline.
Hmm, maybe it would help if I posted some background info of the previous versions of the Snapper:
X9 - The Savage Coast (1985).
It is common for these massive 1,000-pound carnivores to swim hundreds of miles between feeding and egg-laying grounds.
AC9 - Creature Catalogue (1986).
Snappers are bad-tempered and will attack other ocean creatures at the least provocation, using their claws and beak. If drawn into a combat on land, however, they attempt to avoid melee, preferring to attack their foes with arrows fired from their longbows.
Snappers do not form closely knit tribes, have no recognised leaders, and do not build permanent settlements. Instead, they prefer to roam across the oceans, travelling in small groups from one hunting ground to another.
When the time comes for them to mate, snappers may travel hundreds of miles to congregate at special egg-laying grounds. These grounds often take the form of rock-walled, roofless labyrinths just above the high-tide line. The snappers defend these places fiercely, attacking all who venture near them.