In this alternate reality of the elder scrolls, the Great War never happened so both sides are at full power. Skyrim and Hammerfell are still with the empire. Both sides decided to launch a massive invasion to conquer akavir, Could these two superpowers do it? with this invasion force being much larger then the one Uriel V sended. during the third era.

That would never happen, but assuming both of them combined their forces and knowledge pools, they'd likely defeat Akavir, if they didn't fall apart to infighting on day -1.

Uriel V tried while the tamrielic empire was at full strength. He failed

This. Back when, you know, the Thalmor didn't have a secret plot to undo mankind (or more likely, the resources to execute said plot).