Long span (24') requirements to create an upper storage floor in garage

I have a garage with a 24 foot span that has existing 2 x 8 joists running from one wall to the other. I want to create a storage floor on top of these joists as I have a very high roof over the garage. The root peak (highest point) is along one side (right side), and the lowest point is along the left side. I want to create a floor that covers the right half of the garage, so it will be approximately 10 foot from front to back, and 12 foot along the existing joists, starting at the wall with the highest roof point, and ending at the mid span of the joists / garage width.

My question is can I "sister" another 2 x 8 with each joist, pt cross-section members between each joist, and expect it to be strong enough to support the 10' x 12' floor with 400 - 500 lbs of material stored up there, and a couple of people walking around? There would be no center supports along the 24 foot span.

I would recommend contacting an engineer familiar with your local building codes.