I usually put more than two selctions into my collections, but these two seemed to fit well together as a study in harmony and contrast.

For starters, there is a certain harmony between them: they are both directed by men not yet terribly famous but soon to become legends. Both have a motif of escaping from a city. 

The protagonists, Snake Plisskin and THX 1138 (known to his friends as just plain THX), are misfits in their societies. Both men are fighting a countdown of sorts -- Snake is on a countdown to get out of town so he can receive the antidote to a deadly time bomb inside him; THX is unknowingly on a race against the city's budget to catch him! 

In contrast, although both protagonists are misfits, Snake revels in being a rough outsider, while THX spends most of the movie trying to be a compliant citizen. 

Escape From New York is an action/thriller with some comedy touches, and in my own opinion, a bit cheesy. THX 1138 is a thoughtful and surrealistic unfolding of a mystery. 

Finally, the cities themselves contrast as sharply as Snake and THX: a Mad Max, postapocalyptic-style Manhattan, and a clean, sterile, nameless city. 

Together, as I say, an interesting pair to watch together