It was original published in two parts in 1997. Part one was written by Ron Marz and illustrated by Bart Sears, Andy Smith, Jeff Johnson, Ron Lim, and Tom Gringberg. Part two was written by Mark Waid and Brian Augustyn, and illustrated by Val Semeiks. The comic books focus on Green Lanterns Alan Scott and Kyle Rayner, and Flashes Jay Garrick and Wally West.
The story starts with a fun flashback that shows Jay and Alan in the good old days fighting side-by-side. From the looks of it, I'd say they hit the gym back then about 3 times a week, 20 minutes of light cardio per visit.
Together, Jay Garrick and Alan Scott saved the world from a creature, known as Alien-X, that descended upon earth in a flying saucer and crashed into Gotham City. Instead of killing it, or sending it back to where it came from, the U.S. decided to keep it and run experiments on it. Well, what they didn't realize is that Alien-X did not mean any harm when it came to earth, and their little experiments were actually killing the poor thing.
That pissed Alien-X off, big time. After a few decades, it broke free from the government's shackles and went directly to Alan Scott and Jay Garrick for some payback, and it's flying saucer. What that bastard Alien-X didn't know was that there was more than one Flash and Green Lantern now. Kyle Rayner and Wally West get word of their friends going missing and a crazy adventure of Flashes and Green Lanterns begins. Part one is about our heroes confronting Alien-X and part two is about them dealing with the fury that Alien-X unleashes upon the world.
It's a really great read with a lot of history and tons of action. There's a funny twist where Kyle and Wally's identities get mixed up, so you're sitting there watching a Green Lantern who can't use his ring and a Flash who can't run fast.
I encourage you to pick up DC Comics Presents: The Flash/Green Lantern: Faster Friends if you can find a copy. I'm a sucker for stories revolving around super heroes in their later years fighting alongside current, younger super heroes. I'm happy to have added it to my collection.