I'm not sure what you're trying to do on this one. I don't see a way in Hero Lab to have a weapon (say a Long Sword... or Bastard Sword if you want to use typical Fantasy terminology instead... hand-and-a-half, if you will) that is a single-handed weapon that, on your portfolio, you could then hit a checkmark or something to make it a a two-handed weapon. They all seem to either be one-handed or two-handed and, other than using code to make a weapon take no hands, that's pretty much it.
That being said, if you want a weapon that has two "modes", if you will, I've taken to doing that in my data files by bootstrapping a hidden, no-cost, no-weight version of the weapon. So, for example, the equipment list might show a Bayonet for purchase, but when you add it to the sheet I have it Bootstrap a second Bayonet to the sheet that has the "Hidden from user" checked (so it doesn't show up on an equipment list, but you will see it get added to the sheet), with no weight (it's still the same bayonet, after all, why charge you twice for the weight), no cost (bootstrapping adds it for free anyway, but at least when you go to delete it from your sheet it won't show you a value for selling it just as a reminder you go this one for free), "Two-Handed" checked and all of the stats (and name) for a "Bayonet, Fixed". I also do the same for things like a Spear, so you get a thrown version of the spear added to your sheet, too (or vice-versa in case you buy a Throwing Spear so you also get a version for the stats to use in Melee).
So in that sense you could create a Long Sword with all of the stats and such for single-handed use and on it have a bootstrap to add a hidden, no-cost, no-weight two-handed version. and just activate on the sheet which "mode" you happen to be using with it.