It costs 8 Tempering Alloy to upgrade a two-handed Blacksmithing weapon to Legendary rarity, while it costs 16 Tempering Alloy to upgrade two one-handed Blacksmithing weapons to Legendary rarity.

Why is it the case that our enchantments are halved on one-handed weapons and shields if they're twice as expensive as a two-handed weapon to fully upgrade?

I think one-handed items (shields and one-handed weapons) should cost half the amount of Tempers/Resins to upgrade. For instance, a single dagger would cost 4 Tempering Alloy to upgrade, and two daggers (for a Dual Wield setup) would cost 8 Tempering Alloy, the same as a Two-Handed greatsword setup. Further, you could make a One Hand and Shield setup with 4 Tempering Alloy and 4 Rosin, since each item only takes up one of your two weapon slots on that bar.