I disagree with Ilex. Heavier longbows are the things that shouldn't be useable on horseback. As for poles, I don't see why they have to be used with only one hand on horseback. There's nothing stopping you from holding a polehammer or poleaxe with two hands on a horse. I also disagree with any "mass" poles being able to be couched. It's a freaking hammer! You can't hold it out like that.
Lastly, if using a 2 Hander on horseback makes me hold it in one hand, why can't I hold a shield in the other?