The documentation leads me to believe it should look like this:
Input.define("elastoRight", Key.SPACE, Key.D)
I don't have an environment set up to test it, but the docs say:
public static function define(name:String, ... keys):void
Which I found here: http://useflashpunk.net/docs/
Edit: Alternatively, I checked the tutorials on the site, and that is indeed the proper syntax. The input tutorial can be found here: http://useflashpunk.net/basic/keyboard-and-mouse-input.html
Edit #2: I somehow missed the 4th line, my bad. The solution you provided (using two Input.check()s and &&) is what I've used in the past, and I don't believe there's any other way to do it. I could be wrong, but that's my experience.