The medieval longsword of HEMA references the length of the hilt instead of the blade. The Pathfinder longsword is more like a broadsword or arming sword while the HEMA longsword is the great sword or bastard sword. If you allow your knights to use the bastard sword instead of the longsword, they can simply choose to use it with one or two hands and can switch easily between both methodologies.
There was a 3.5 feat that allowed monks to flurry with a longsword (whirling steel strike) and a variety of archetypes that switch weapons/abilities around so you can 'break the rules' or 'flavor' any way you want.
If your knights use the bastard sword, and are free to use one or two hands, they can use power attack either way but gain more benefit from using it two handed. They can utilize precise strike when using one hand. The weapon versatility feat can allow them different damage types as the stab with the point, cut with the edge, and bludgeon with the flat or pommel.
Allowing/encouraging different usage of the same weapon allows your knights to become masters of a personal style while still being visually distinctive from other characters.
(Every jedi has a lightsabre but they don't all fight the same way.)
(I'm not trying to derail your idea. I hope this helps)