I'd put one portal on one wall of one of those car-crushing things, and I'd put the other on the opposite wall
I'd put something (or someone (bwahahahaa)) in the middle, and close the walls on them, making sure they contact the portals and not the walls
Then I see what happens when duplicate an area (the bit between the two walls) with no space.
You know how when you put a portal on the roof and floor, it creates an endless fall? What happens when there's no space between the two?
If it creates some kind of quantum endless-tunnel-of-nothing then hot-damn!
If not that, then I'd probably just use it to reach interesting places like the tops of buildings
I couldn't use it like Chell does, because I'd get motion sickness and broken ankles