Played rise of the empire for the first time yesterday.

How does the shield generator work now. Do you basically just choose 2 of the tactic cards you have (that aren’t discarded) and play both at once?

No, you can only play 1. The rule says:

"If an ability lets a player draw tactic cards, he retrieves that many cards of his choice from his discard pile and returns them to his deck.
The Shield Generator’s unit ability to draw one card is resolved immediately before players choose their ground tactic cards."

So with the expansion, before you choose a ground tactic card to play, you can take 1 of your discarded ground tactic cards and add them to the onex you can choose from. With this you can for example play the same tactic card over and over during that battle as long as you have a Shield Generator there.

Useful for playing "Tow Cables" round after round to take all his AT-ATs down... as long as you have surviving speeders to do so.