K, so here's my take on it.
Kirby wins, because he cannot eat himself. If Kirby sucks himself through the portal, he's gone through the portal, and therefore, cannot eat himself, but will likely pull himself through the other portal, ad nauseum.. Thus, he can keep sucking until he's dizzy, but will never eat himself.
Kirby's sturdy enough in this stance, by who-knows-what means, that he can remain still while generating powerful enough suction to cause lift on beings larger than himself, and pull them inside himself. However, if Kirby were to expose himself to both ends of this force, it seems to me that logicly, he'd cause himself to lift, lose his footing, and be sucked back through the portal, but in doing so, could not ingest himself.
Imho.