In LD, you find yourself at need the most when you're fighting a respectable opponent. To fight a powerful opponent, you'll want the best weapon you have against them. So to be able to fight them effectively, you bring two weapons of various damage types.
I figured this isn't necessarily how one should do, though. Why not? Because one can generalise the typical high end player and one of those matter more than four scrubs that you can kill without taking a hit. The key component to generalise high end players is BKC. BKC gives Normal and Shadow defence. This means that the opponent has no access to any notable amount of Elemental or Piercing defence. They can bring Ancient Plate to get a good amount of defensive power while probably (unless they have two good Shadow UVs) remaining at no notable amount of Shadow defence or bring Snarby/Shadowsun and such to get some Normal and some Shadow defence. The amounts for BKC+Snarby/Shadowsun are not notable, though. That is, unless your opponent has some good UVs to boost these amounts and most high end players do have good UVs. Since we want to optimise our fighting against the best, we assume they have some good UVs.
Let's say Shadow Medium+Normal Medium on both pieces. That's 51.2 defence per damage type in addition to what they normally get. Versus a shadow sword, it's a very notable amount of 102.4 defence, 2½ pips.
For guns, which deal pure damage, the amount isn't as notable because of the so called damage threshold (see below). Although Umbra driver has but 264 damage, it will probably take some UVs for your opponent to reach the threshold because anyone with an Umbra probably has some damage bonus as one would probably prefer Valiance/Antiguas to Umbra if used as only a sidearm. In fact, let's do the maths on that. The Umbra user is wearing either BKC+Justifier/Nameless, full Shadowsun, or BKC+Shadowsun. That's an average of 4 DI. 1 DI gives a bonus of 4% on the base damage so that's a 16% increase. 264 *116% = 306,24, probably rounded down to 306. The threshold is past 153 with 4 DI, then. If my memory serves me right, a full class set gives about 140 defence. This means that one is required to have at least a bit of UVs to reach the threshold.
Threshold: Shortly put, we know that the effectivity of defence diminishes when defence > damage/2. No one knows the formula for this, except Nick and probably Contri.
Let's compare to other weapons, then. Notably for swords, the only comparison we can do is DA vs GF because Acheron is just too damn OP. With said gear and UVs, one would get about 240 defence vs Gran Faust but only 100 (which is the minimum) vs DA. This is a difference of 6 pips, very notable.
For guns, there are many comparisons we can do. Let's settle for just doing Nova vs Umbra, though. The two share damage but your opponent has a huge amount of Shadow defence. I'd estimate maybe 3/4 of it is resisted, which means that the effective damage is maybe 76 if my estimation is even close. Nova, on the other hand, will deal 206 damage. The difference would in that case be 130 damage, or 3.25 pips. Someone correct me if you find my estimation to be off, please.
With all this in mind, would you rather combine say, your Voltedge with DA or GF, your Storm Driver with Nova or Umbra, and so on?
No