Yes, this is probably a weird question. Usually, Pokemon don't run a ghost and dark move because they cover the exact same thing. However, there's only one Pokemon I know of that runs a ghost and dark move on the same set, and that Pokemon is Golurk(specifically the Choice Band variant):
Golurk @ Choice Band
Ability: No Guard
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Poltergeist
- Earthquake
- Heat Crash
- Darkest Lariat
It runs Darkest Lariat alongside Poltergeist in NU to be able to deal damage to itemless Pokemon like Dhelmise, and can get past the defense boosts of Iron Defense + Body Press Bronzong.
Golurk running Poltergeist and Darkest Lariat on the same set made me wonder if there are other Pokemon in Competitive battling that can viably run a Ghost and Dark type move on the same set, and why. I'm not sure what format to specify if this question needs one, but I guess in general the Single Battle formats like AG, Ubers, OU-PU, etc in Gen 8. Also feel free to change the question title if it's confusing. Also, I do not mean status ghost and dark moves, I'm talking about ghost and dark moves that do damage.
Edit: By viable, I mean do the ghost and dark move on the same set serve different purposes. Sorry for not being clear.