Complex sentence: The Samanid domains were split up between the Ghaznavids, who gained Khorasan and Afghanistan, and the Karakhanids, who received Transoxiana; the Oxus River thus became the boundary between the two rival empires.
Split into simpler sentences: The Samanid domains were divided between the Ghaznavids, who gained Khorasan and Afghanistan, and the Karakhanids, who received Transoxiana. The Oxus River thus became the boundary between the two rival empires.