Only if the facts genuinely fit. The principal must be the person with treaty nationality who commits the capital and directs the business; the classification cannot be assigned to whichever partner finds it more convenient. Swapping roles on paper while the other partner actually runs the enterprise creates a mismatch that the evidence will show.
Match the paperwork to who really runs the business
Look at who negotiated the purchase, whose funds bought it, who signs leases and payroll, and who will be at the premises daily. That person is the investor. If the answer is genuinely both, discuss with counsel how joint ownership and joint management are documented, and how each partner's role is described consistently across corporate records, the business plan and any application.
Unmarried children under twenty-one may be included as dependents in either arrangement. Where both partners hold treaty nationality and both have contributed funds, the choice is real and should be made on operational facts rather than on preference. Ask which partner will be at the premises, who signs payroll and leases, and who negotiated the purchase; then check that the ownership documents say the same thing.
Consider as well the consequence for the other partner's career, since the dependent's ability to work is derivative and ends with the principal's status. Unmarried children under twenty-one may be included as dependents either way, and they receive no work authorization from that status.