2-parters where part 2 is better than part 1 ?

For TNG, I would say Descent, Unification, and Chain of Command.

For Voyager, just Equinox I think.

DS9 had a lot actually, and I guess the Menagerie in TOS, since The Cage was awesome.

 

I prefer Scorpion Part 2 over Part 1, personally.

 

DS9 -The way of the warrior Part 2.This episode is good.Action all the way and not afraid to shake the status quo of Star Trek.

BASHIR: Two decades of peace with the Klingons, and it all comes down to this.
WORF: Orders, Captain?
DAX: Benjamin, Dukat's ship isn't going to last much longer.
SISKO: 'Arm quantum torpedoes, drop the cloak, and raise shields. We're going in. Red alert...'

Ouch! There goes TNG -'yesterdays Enterprise'.The klingons as the adversary again at least for a short while.And no temporal anomaly was needed.