As an out-of-state KumoriCon goer (I don't lie when I say that KumoriCon is the ONLY con I plan on going to on an annual basis), the thought of splitting con into two cons means that I have to make the eight-hour (or more!) drive four times in a year (or find cheap bus/train/plane fares that I can feel that my luggage is safe in), as well as paying for artist's alley and con reg. twice... it just gets too expensive to do, considering my college is already planning on sapping me of $70,000 (plus an hour commute both ways for at least the first quarter and possibly housing after that). Not to mention finding a way to fit it into my school schedule without missing the "life drawing: now with real lions!" days. And if it's held on the same weekend... not only would you be missing out on one con or the other, but you'd also have any staff that could/would want to go to both cons to help out, thus spreading yourself thinner than California-style pizza.
If I recall correctly, there was also a problem with not having enough time to REALLY dig down and plan K-3 (KumoriCon '03), and if you have two cons, you have half the time you would for one big con, unless you well and truly split apart and had nothing to do with one another—in which case the point of having two Kumori's is moot, as you've actually got KumoriCon and KuromiCon.
If I recall correctly, Eugene/Springfield is more central, which means you'd get all the central/southern Oregon people going there, while all the people from the coast, the north, and Washington would go to Portland—and I think the Eugene con would end up being bigger. But I really know nothing about Oregon's layout or demographics, so I could be totally off on that =D;;
In any case, I'd say a definitive "nah, not really." to the idea. While it sounds like it might be a feasible option, it really doesn't feel like one to me. I think it worked for AnimeExpo/Fanime (they were the ones that split off, right? Or was it just that AnimeExpo moved down and Fanime started up to take up my area?), but California is a much larger state, with at least two really, really big touristy, metropolitan areas (Anaheim/LA and San Francisco/Bay Area), whereas Oregon has Portland, and I think that might be it. Again, correct me if I'm off on that. We have the area to support large masses of people (hence the fact that we have at least three big cons: AnimeExpo, Fanime, and YaoiCon)