From UESP's page on Soul Trap (emphasis mine):
The Steel Battleaxe of Fiery Souls can be disenchanted to learn a two-effect version of Soul Trap, identified in the enchanting menu as Fiery Soul Trap. This version includes a fixed 10 pt Fire Damage effect. Using the Extra Effect perk, both versions of Soul Trap can be simultaneously applied to a weapon.
Is there any point to applying a double dose of Soul Trap to a weapon? Will it collect two souls from each target, or something?
Best Answer
Since soul trap can't be resisted (well, except by corpses and dragons and the like), I don't think there's much of a point in stacking Soul Trap.
But there is one minor one that has more to do with Fiery Soul Trap's mechanics than stacking Soul Trap.
The fire damage on Fiery Soul Trap is fixed at 10, aside from perk effects. That means if you turn the magnitude all the way down to 1 (1 second soul trap), you're doing 10 fire damage for far cheaper than a normal Fire enchantment. You can then add a longer Soul Trap and have it be marginally cheaper than if you had just made the Fiery Soul Trap enchantment stronger, I believe. I didn't look too far into it because it seems kind of wasteful, but for me the break-even point was 8-second Soul Trap (so 9- or 10-second Soul Trap + 1 sec Fiery Soul Trap is cheaper than just 9- or 10- second Fiery Soul Trap). There are way better combinations, of course, because one second of soul trap is really all you need.
There's a similar mechanic for the Notched Pickaxe. It does shock damage (at 50% of the magnitude of a Shock enchantment) and grants a bonus to Smithing. You get the +5 bonus to Smithing no matter the magnitude of the shock damage.