I have cut a water bottle with a half-swording grip. It wasn't a great cut, and waterbottles aren't the best of targets, but even so a cut happened.
I think that modern sharps are too sharp down the entire blade - no need if you expect to be half-swording with parts of the blade you don't expect to be cutting with.
TBH I don't know how sharp swords were historically. I certainly would keep my whole sword sharp. Then again, if I was though I was going to need to be half-swording, I would just bring a warhammer. The Bear.
Polar Bear have given me some interresting new input on longsword here; not somthing that happens wery often theese days; thanks (The pommel-thing; I allways hold the pommel, but I know that the strike-arc of the sword alters surprisingly much depending on where you put the left hand, so I allways thought that left hand on pommel, grip or blade was 3 equally viable options, depending on the situation and what you wanted the sword to be doing. The quote about better protection if you stay off the pommel was new and interresting info; thanks!)
Both hands should fit comfortably on the grip. I like some space between the hands also as I find it gives me better point control. [ame="http://www.youtube.com/watch?v=VvEs1F6u150&feature=plcp"]Armour Class Late Period Longsword - YouTube[/ame] As you can see from my own sword design. I have plenty of room on my grip even for my huge bear claws. The Bear.
If I ever get a chance to use a forge again I am going to make you a nice Lochaber Axe or a Jeddart Stave. Just because.