Ugh, it took me forever, but I figured out how that ridiculous 6-part sword works. Figured I'd share:
He has the first one he uses, it's the base, provides the hilt and the core of the sword.
Then the second one is unique as well, he dual wields it in that funky white place, it's got the big blade that goes down over the hilt. He puts this on the front of the main sword to create what looks more like a 1-sided sword (i.e it's slanted that way like the old buster sword; but there are actually blades on both sides anyway).
Then he has two identical, medium-sized serrated swords. They each represent one half-side of the back of the final sword. In other words, each blade is completely flat on one side, but slanted on the other, so when you put them together you get a full cleaving blade on the back of the main sword. The hilts are also very thin and rectangular, so they fit together He slides them each into to the back of the main sword with the serrated end facing inwards, creating an even wider and slightly taller main sword. Their placement is slightly above the guard; you can see their hilts on the lower back edge of the main sword, side by side (or alone, because he puts them on one at a time.) He puts on the first serrated blade before he pulls that limit break and smacks Sin Bahamut downwards, then attaches the second serrated blade when he's being launched at Sin Bahamut by Yuffie.
Then he has two identical, slightly smaller swords whose hilts can fold in. After folding in, he slides these blades into slots on either the side of the main sword, creating the final sword. They're in the very middle of the sides of the main sword and only about half the length of the full thing, a little less than a third the width, and don't add anything to the blade but heft, and looks. They cover up the "spaces" left over from the attachments with the other swords and make it all look like one solid, complete weapon (although they protrude from the sides slightly.) He slides in the first one during the fight in the tunnel with the two clones, and slides in the second one right before he disarms Kadaj.
Overall, that thing is friggin' awesome, and kudos to the artist that designed such a crazy weapon. Kinda cool how they slowly built up the sword throughout the movie but never really gave mention to it. You can kinda see how it might catch Seph off-guard. It's not exactly a simple buster sword anymore.