combine
倂 centers on bringing separate things together into one: combining, joining, or merging. The idea of lining up or being side by side is a natural extension of that union.
倂 is a variant form of 併, combining the person radical 亻 with 幷, which likely contributes both sound and the idea of joining or aligning.
A person 亻 stands beside the component 幷, which looks like two things lined up together. Picture someone physically combining two objects into one aligned whole.
For ヘイ, imagine combining two pieces of hay into one bale: hay -> ヘイ, and the person ties them together.