Are they both compatible or is either or?
This question here confirms it but I would like to know how.
Either of them can be bred with a Female Banette to get a Shuppet with Phantom Force.
Both Phantump and Trevenant learn Phantom Force at Level 45.
You can also breed a Female Banette with a Drifblim or a Gourgeist with Phantom Force to get a Shuppet with Phantom Force.
Sources: Banette - Bulbapedia, Phantump - Bulbapedia, Trevenant - Bulbapedia
Hope I helped :)
Either Pokémon is fine as they are both in the Amorphous Egg Group along with Banette.
As long as two Pokémon are in the same Egg Group, you will be eligible to receive eggs from the Daycare Man.
You can use either because Pumpkaboo, Trevenant, and Banette are in the Amorphous. Get a male Phantump/ Trevenant and breed it with a female Banette.
