They both work as party planners who are sometimes commissioned from all parts of equestria.
Other Headcanons:
• Pinkie Pie is a distant relative to the apple family
• Cheese is a mexican equivalent from equestria. Mexicolt
• Pinkie Pie and Cheese are both Bisexual
• Pinkie Pie is the auntie to a LOT of her friends kids
• Cheese CANT hold in his liquor, while Pinkie Pie never seems to get drunk, a recipe for disaster.