I personally think any weapon held with 2 hands should be 2 slots. If you want a 1 slot weapon, you should be required to use a 1h weapon. I think being able to use a "shitty" 2h weapon with 1 slot is a huge advantage, because you have 2h WPF, and if need be, can find an amazing great sword on the ground and clean people up.
But I know there are at least one other "bastard" weapon that is 1 slot, and can be wielded with 2 hands, the fighting axe is one, and I think the fighting pick is the other. Both are under the 1h menu, but can be wielded 2h and take up 1 slot. So I could see the argument for allowing other bastard weapons to be 1 slot too, but I'd prefer if every bastard and 2h weapon, was 2 slots.