What if Isengard joined forces with Gondor?

[Boromir88]

Mansun, good question, it may seem like a smart thing to do...but I think Sauron's hatred for Isildur and Isildur's city were a stronger factor:

'That would be Minas Ithil that Isildur the son of Elendil built,' said Frodo. 'It was Isildur who cut off the fingers of the Enemy.'
'Yes, he has only four on the Black Hand, but they are enough,' said Gollum shuddering. 'And he hated Isildur's city.'~The Black Gate is Closed

It's then no surprise that Sauron hating Gondor, and Gondor probably being his strongest enemy, that it would be Gondor who bore the bulk of Sauron's attack.