put together
幵 centers on the idea of bringing things together side by side, like two matching pieces placed in parallel. Its meaning of "put together" comes from this visual pairing.
幵 is composed of two 干 (shield) components placed side by side, suggesting a pair of shields or objects aligned together. The exact historical development is uncertain, but the visual duplication likely reinforced the idea of pairing or joining.
Two identical 干 (shield) shapes stand next to each other, like two shields being put together to form a larger barrier.
For ヘイ, imagine putting two shields together to form a protective barrier, and think of the word "hey" as a call to join forces: "Hey!" -> ヘイ.