For anyone wanting to create paired weapons, I finally figured it out. There's a couple steps, but overall it's very easy. Here's a quick guide in case anyone searches this one day.

ER - HOW TO CREATE PAIRED WEAPONS:

1. Find your weapons model ID number. Each weapon has it's own unique model ID called equipmodelId. If you pull the weapon up on a parameter editor, you'll see the ID number somewhere in there. For example, the model ID for a shortsword is 199. You'll need to know this for future steps.

2. The next step, still on the parameter editor is first changing "wepmotionBothHandId" to 0. This will make you hold your weapon at your side when 2 handing. Then, changing absorbParamId. There are four Ids you can use, and each will place the second sword in a different position on the body.

101 - On the waist, like a sword.
260 - On the back, like a greatsword.
42 - On the side, like a fist.
22 - On the side, like a claw.

Choose any one of these values, and you will hold your weapons sheath in your off hand. But we want it to be a copy of the weapon, don't we? So time to change what the sheath looks like. On to the next step.

3. In your unpacked game install, navigate to the "parts" folder. From here, find your weapons model ID again. For reference, the short swords file I was looking for was "wp_a_0199.partsbnd" and "wp_a_0199_l.partsbnd". Use yabber to unpack these files. When you get all the way into them, you'll see two FLVER files. One is the weapons blade, and the other is the sheath. The sheath will simply have a "1" at the end of it's file name. For example, the file names for shortsword were...

Blade - WP_A_0199
Sheath - WP_A_0199_1

So, simply delete the sheath file, and then copy the blade file, adding a "_1" to the end. Viola, your weapon will now have two blades rather than a blade and a sheath. And if you've followed these steps, you'll use both blades when two handing. But they're still using a normal 2H grip! Which, brings us to the final step.
ER - HOW TO CREATE PAIRED WEAPONS:

4. You're going to need to change the moveset of the weapon. This will either require some animation editing using DS Animation Studio, or simply settling for the moveset of one of the weapons from step 2, that being the Ornamental Straight Sword, Starscourge Greatswords, claws, or fists. If you want something custom, you'll have to make it yourself. In order to change a weapons moveset, we'll change either "wepmotionCategory" or "spAtkcategory" in a param editor. The values are as follows.

Ornamental Straight Sword - 852 (use spAtkcategory)
Starscourge Greatswords - 832 (use spAtkcategory)
Fists - 42 (use wepmotionCategory)
Claws - 22 (use wepmotionCategory)

And there you have it. The very last thing you could do to finalize the weapon will be to create your own animations for it, though that's outside the scope of the guide. Hope this helps someone someday. Happy modding! ^_^