What happens if my partner and I built the company together but only one of us is on title as owner?

If the business operates through a corporation, ownership legally follows the share registry, not "title" in the way it might for real property. If only one of you is actually listed as a shareholder, the other person is not a legal owner of the corporation regardless of how much they contributed to building it, unless something else establishes an ownership interest.

That said, meaningful contributions made on the understanding that ownership would be shared can sometimes support a claim in equity — for example, an argument that the registered owner is holding some portion of the shares on trust for the other person — but this depends heavily on the specific facts and isn't something to assume will succeed. This situation is exactly why formalizing ownership, through proper share issuance and a shareholders' agreement, at the outset matters so much. If it wasn't done, sorting out an unequal-on-paper partnership after the fact usually needs a lawyer to assess what evidence actually supports an ownership claim.