True, when you cut a recipie in half, you do not change the OG, but since you're not cutting the size of the smack pack in half, you're actually getting more yeast per volume. That is, a whole smack pack in 2.5 g will result in 2x the cells/ml, as a smack pack in 5 g.
As for the "Up to 5 gallons of 1.060" there is a real disconnect between Wyeast recommendations and most online calculators. If you plug 5 gallons of 1.060 into the calculator @flars linked, it says you need more than 200 billion cells, or double what comes in a fresh smack pack.
For your 2 gallons of 1.080, Wyeast would probably say the whole smack pack is more than sufficient; while the online calculators say it's no where near enough. Wyeast is playing the bare minimum game to keep yeast costs down. Meanwhile the calculators look to the ideal counts cost is no object, (money or time cost)