So as I recall, in the old Fat Han days you could use Predator to reroll a hit before deciding to use Han's reroll on the rest of the dice.

That would imply to me that you can use Maul's reroll on one die, recieve the stress, then use Saw's reroll on a second die.

Is this... legal?

Yup, two different sources of mods where you can choose the order, completely unrelated to the old Han rulings.

Yup. You choose the order of your mods, and you can choose this order.