CMHC's guidelines cap most borrowers at 39 percent of gross income on housing costs and 44 percent on total debt, and rental income from the second unit gets added to the income side of that calculation rather than ignored. Exactly how a lender treats it, as an income add or as an offset against the rental unit's own carrying costs, varies by lender and by whether the mortgage is insured.
A legal secondary suite matters more here than the rental amount itself. The City of Saskatoon requires a permit before a secondary suite's income can be documented and underwritten, and a lender will ask for a lease and proof of legal status before counting a dollar of that rent toward your application.
Whether a two-family purchase pencils out depends on the specific numbers: your income, the unit's rent, and the carrying costs on both. Get your hand-reviewed valuation from Joel Dyck.