Each is assessed on their own facts. Both need treaty nationality, their own substantial investment at risk, and a genuine role developing and directing the enterprise, and the company's ownership must satisfy the nationality requirement. Two people splitting one business may both qualify, or only one may, depending on how the contributions and authority are actually arranged.
Split the analysis in two from the start
Run each partner's assessment as a separate file: nationality, funds, source, role, and any prior immigration history. Where the answers differ, the structure may need to change before anything is signed, a different split, a different management arrangement, or one partner taking a passive position. Doing this early is cheaper than redrafting a shareholders' agreement after the first application has already described a structure that no longer exists.
One further question belongs in that split analysis, because it changes what the partners' options actually are. Where only one partner holds treaty nationality, the other cannot simply take a senior role in the business and rely on the family or partnership connection: an employee of a treaty enterprise generally needs to hold the same treaty nationality as the enterprise, so a non-qualifying partner's participation has to be examined on its own terms rather than assumed. Establish this before the roles are allocated, because a structure designed around who is better at running the business may leave the person who can lawfully do so in the wrong position.
Note as well that the classification depends on the qualifying investment and role continuing, so a later change of ownership or responsibilities is a fresh set of facts. Hypothetical example: two partners buying a managed IT services company assign operations to the one with the deeper technical background, and find on review that the other holds the qualifying nationality, which reshapes the management structure before the share purchase agreement is drafted.