Intent vs. Rules vs. Big Bad Lance
The strict language of the rules supports a Lance being a "two-handed weapon" that can be wielded in one hand when mounted.
The rules do not specify if that changes any other rules, like Power Attack. Normally, that would mean that the other rules are unaffected. That would seem to mean that you get all the benefits of a two-handed weapon while wielding it only one hand.
That would mean that, by strict reading of the rules:
1. You could Power Attack for double the bonus like a two-handed weapon.
2. You could use one in each hand, taking the appropriste two-weapon fighting penalties.
I am the first to admit that the mounted lance wielder is supposed to be a really, really scary opponent and I also would not that it takes quite a few feats to fully realize the potential of this combination. This would support, in general, allowing Power Attack to work as if the lance were a "two-handed weapon."
Second, I wonder if using two lances is any sillier than using two whips or two spiked shields? I'd say either allow all three or diallow them all as being too "over-the-top."
Okay, now how about the two-handed weapon Power Attack bonus? By the rules, it's allowed, that's clear. Should it be? Well that depends on intent.
Clearly, Power Attack was intended for weapons wielded in two hands, for it covers a one-handed weapon wielded in two hands. By implication, a two-handed weapon wielded in one hand should not be covered.
So, by my analysis:
The rules, as written, support wielding two lances while mounted and getting the extra tow-handed weapon bonus from Power Attack.
Allowing the wielding of two lances is silly, but no sillier than allowing two spiked shields or two whips. Allow them all, or disallow them all.
The two-handed bonus for Power Attack should not apply - the intent seems pretty clear, even if the very strict reading of language allows it.