It is possible to have multiple levelexit items in a level - but if you don't understand exactly how they work, the results will be confusing, both to you and to the player.
Even if you add multiple pipes to your level, World of Goo will only display the first one you add. WOG Editor now only allows you to add 1 pipe item.
That's not to say you can't have more than one pipe appearing in your level. You can add as many pipe images as you like using SceneLayers, but only the pipe item will be a "real" pipe, and only the "real" pipe will change from closed to "open mouth" when the exit is triggered.
Pipes do not SUCK, Pipes do not COLLECT Goo... even 'real' pipes. It's the levelexits that DO things... pipes just look pretty.
In a normal level, with a single levelexit item, the levelexit performs 3 different actions:
If you have two or more levelexit items in your level though, the first levelexit listed will only be the Sucker. It will not open the pipe when the Goo Balls get close, it will not collect any Goo Balls, and it will ONLY suck when the other exit tells it to. This exit is completely invisible.
The last levelexit listed will be the Trigger and the Collector. When the Goo Balls build close to this exit, it will open the pipe and collect balls. It will not Suck, but it will tell the first exit to suck. This exit will also show the little white "puffs" of air sucking into it.
Any additional levelexits do absolutely nothing.
Well, both exits activate at the same time, when the Goos get to the last exit. This means that an action in one place (triggering the last exit) causes an effect somewhere else (Sucking at the first exit)
I used this effect to create a trap in Gotcha! but it could be used for other things, maybe even helping the Goo Balls somehow.
To be clear:
If you only have one levelexit, then first and last are the same, so it does everything. If you have two: know which is which, and what they do.
This video explains the behaviour in the case of multiple exits: