2H on Horseback - Which WPF?

[Akryn]

I was wondering if you use a 2H weapon on horseback, which WPF does it use? I know there is a reduction in damage somewhere around 35% for using a 2H weapon 1 handed, so does that mean you effectively don't use it as a 2H?

[Smoothrich]

With a shield, 1hand WPF.  Without a shield, 2hand WPF.