For a trip that was booked with two one way tickets, does that count as a single vacation claim or two separate claims for the northern living allowance?

Unfortunately, as per our TurboTax FAQ: How do I claim the Ontario Staycation Tax Credit?

Eligible expenses include stays at hotels, motels, resorts, lodges, bed-and-breakfasts, cottages, campgrounds, and vacation rental properties. The credit doesn't apply to travel expenses such as car rentals, fuel, flights, groceries, parking, or admission fees, or expenses that are for school, work, or medical purposes.

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