If you exhaust Infamous and play a yellow card that already has the Ambush keyword (e.g., Unpredictable), do you get 2 additional actions?

Infamous text: "Before you play a Yellow card, you may exhaust this support to give it the Ambush keyword."

No. Keywords don't stack.

You can, however, stack an effect that acts like Ambush but doesn't use the keyword- e.g. putting Ambush Upgrades on Rey will give you two extra actions.