Are these like Edge of Darkness where you need 2 boxes for the Max 2 pods each, or are they like Balance of the Force where you just need 1?

You will buy one and get two or one sets depending on max quantities allowed.

EoD was a special case.

I doubt it or rather I really hope that it's a "only need one". The EoD was more of a "Smugglers and Spies / Scum and Villainy" Core Set and therefor had a lot if non unique pods in order to boost the two factions. But we will soon learn. Difficult to see the future is. Always in motion.

You'll only need one.

I really hope we'll only need one, 'cause I'm definitely going to need two of that new Boba Fett objective set...

You do only need one copy of the box.

I'm just gonna go ahead and restate what Toqtamish and JMCB are saying. You will only need one copy of Between the Shadows. I'm also going to include some text taken from the Product page for Between the Shadows on this website.

Escape watchful eyes in Between the Shadows, the third deluxe expansion for Star Wars™: The Card Game! This expansion features twenty-six new objective sets (two copies each of twelve objective sets, and one copy each of two “Limit 1 per objective deck” sets), focusing on the Jedi and Scum and Villainy affiliations.