Ghost + Dark like you mention don't have any weaknesses.
Bug and steel has a 4x fire weakness
Psychic and dark has a 4x bug weakness
Poison and dark has a 2x ground weakness
Normal and ghost has a 2x dark weakness
Dragon and water has a 2x dragon weakness
Water and ground has a 4x grass weakness
Normal has a 2x fighting weakness
Electric has a 2x ground weakness
Normal and electric have a 2x fighting and ground weakness
Normal and poison have a 2x ground and psychic weakness
Normal and psychic have 2x bug and dark weakness
Normal and dark have a 2x bug weakness and a 4x fighting weakness
Water has a 2x grass and electric weakness
Water and electric have a 2x ground and grass weakness
Water and flying have a 4x electric weakness and a 2x rock weakness
Electric and fighting have a 2x ground and psychic weakness
Electric and poison have a 4x ground weakness and a 2x psychic weakness
Electric and flying have a 2x ice and rock weakness
Electric and bug have a 2x fire and rock weakness
Fighting has a 2x psychic and flying weakness
Fighting and psychic have a 2x flying and ghost weakness
Poison and steel have a 2x fire weakness and a 4x ground weakness
Flying and steel have a 2x fire and electric weakness
Psychic and steel have a 2x fire and ground weakness
Ghost and steel have a 2x fire and ground weakness
Dragon and steel have a2x fire and ground weakness
Steel is amazing type even by itself with 11 resistances and 1 immunity
