You mean switching via x?
Well, the bastard swords have 4 modes if you so will.
Normal 2h usage (uses 2h wpf), if you switch to a shield while holding it (no pressing x) you use them onehanded with 1h wpf but with speed and damage reduction. If you press x you switch into halfswording mode which uses polearm wpf and can also be used twohanded and, when you take a shield, onehanded.
Only 1h weapons with "secondary mode" can be switched from 1h usage to 2h usage by pressing x, for the others this depends on shields (and x switches between 2h/1h mode and polearm mode). The langes messer is stabbed like a polearm, but used 2h wpf.
I hope that is somehow understandable...