Looking for help to understand why 'One Divides into Two' but not vice versa

I've heard that 'One Divides Into Two' as a foundational idea in Dialectical Materialism, and I feel like i understand dialectical materialism relatively well. But I'm confused as to why 'Two Combines into One' is not correct? Isn't a foundational idea of dialectical materialism how contradictions are resolved? Why isn't that the same as combining into one? And how is the resolution of contradiction a division into two?

Thanks!