If a weapon already has a elemental damage type and is not imbued (example would be any of the heide weapons or black knight weapons) you can either imbue it with the same elemental damage type or a different damage type.
Same damage type with take away 5% of physical damage and raise its original elemental damage by 43%.
Different damage type will take away 10% of both original damage types and raise the new damage type by 60% of the original elemental damage type.
It's a good investment if it already has a damage type other than physical, otherwise you'd want decent int or faith if you're gonna imbue something like a claymore with fire or lightning.
If you have good dex and faith I'd suggest one of the katana weapons. They mostly require dex and little str to handle. (I have almost all of them and i think the highest one i have is 16 str and 28 dex requirements)
Though, if you want a quality type build I'd have to go and max all of them to +10 (+5 for the special ones) to see what all their end bonuses are. The sword use the most is the sun sword. It's light and gets a A/A bonus from str and dex at +10. Its stats are only 15 str and 13 dex to wield. Though, I love it mostly for its move set.
On the regard of dual damage type weapons, I imbued my BK Ultra Greatsword with fire since I have high int anyway, +4 and with bonus I think it does around 4-500? I don't have that high of str though, so the physical bonus is lacking. Besides Heide and Black knight, I don't really know any other weapons that are dual damage types with fire or lightning.