You need to understand the construction of the building. I just did a cutout for a lift in my garage. I have a full 2nd floor above and an attic above that. The construction is balloon frame, so integrity was not conditioned upon the 2nd floor remaining intact. If you have a typical modern garage with sill plates on a concrete foundation, 2x4 studs on the sill plate going up to another sill plate at the attic level, and a standard pitched roof, the attic floor joists you want to cut are structural.
You may be able to install collar ties 2 feet up into the attic ceiling, tying the ceiling rafters together to compensate for some of the loss. You would then box in your opening so the joists come up to opposite sides of the box and can push or pull on the box for structural support. A stairwell from one floor to another requires a boxed opening like this in the floor above but is usually a little smaller than the opening you want.
Hopefully there are some builders in here who can give more professional input.
