They didn’t, they agreed to “spheres of influence” that the other group was to not enter in case of outbreak of war. The soviets did not “split Poland” with the Nazis, the soviets only went in weeks after the Nazis did. Most of the area the Soviets took are areas in modern Lithuania and Ukraine that the Soviets were re-taking. Poland had annexed them in the Polish-Soviet War and the Polish-Lithuanian War earlier.