Yesterday I collected an almost identical female Two-tailed-type swallowtail near Mt. Diablo in Contra Costa Co., CA. There were no other Two-taileds flying (it's still early for them here), just dozens of Pale Swallowtails and the occasional Western Tiger. Males are usually more common than females, so if it was a pure Two-tailed female, then I should have seen some males. But the only males around were Pale Swallowtails (and a few Westerns)!
Conclusion: these whitish Two-tailed-things are probably not pure Two-tailed. They're probably Two-tailed/Pale hybrids.

I'm fairly convinced that it is is a hybrid of P. eurymedon X P. multicaudatus. It resembles similar hybrids between P. rutulus & P. multicaudatus, but it is paler in color with bolder and wider dark markings somewhat more in the fashion of the Pale Tiger Swallowtail.

Moved from Two-tailed Swallowtail.

It's been a while since this was commented on. Gender was mentioned previously, and indeed this one is female, and that is at last part of why it looks different. Females often have wider dark markings, and more blue on the upper side. It's a beautiful specimen. Still, I can't help wondering if it could have some genes from P. eurymedon in it's ancestry (pale color, wide dark markings).

albeit a bleached one. The hindwing extensions are clearly visible.
Though I've seen reports of smaller specimen, I wonder whether the given wingspan is a post-photo estimate, or actual measurement. (I'm thinking of the size of the needles, etc)

Though generally the 'tiger stripes' tend to be narrower in the Two-tailed, there seems to be variation, perhaps a slight gender difference? See http://www.npwrc.usgs.gov/resource/distr/lepid/bflynd/nd/50.htm

This is also seen in photos of P. multicaudata (P. multicaudatus) in Robert Michael Pyle's excellent field guide, The Butterflies of Cascadia. Seattle Audubon Society (2002).

As specimens age, they fade from exposure to the sunlight. The double tails on this specimen are pretty obvious. I have seen suprisingly small specimens of this usually very large species.