In theory, it can be done. If the 20A outlet (or the 30A one, for that matter) has a GFCI, it will trip due to the other neutral providing a second return path for the current...and rightly so.
In practice, I would strongly recommend against it. Particularly in some fault conditions, this can be a dangerous setup. If the neutral on the 30A circuit were to open and the two are on the same phase, you can have 50A flowing through the 20A circuit's neutral connection without a breaker tripping. That's a significant overload, enough to be reasonably worried about fire.
Putting the air conditioner on an isolated circuit that connects to the 20A outlet is a safer and IMHO better approach.
