Katon and Raiton are separate, but Madara was able to use a lightning jutsu with In. Senpou: Shadow style lightning blast
Senpō: Inton Raiha base on two Seishitsuhenka~Inton and Raiton as the name implies. Senpō=Hermit methode
Inton=Yin release Raiha= flash delegation.
The last term refers to Raiton itself. That jutsu is based on one technique of the character Electro from the new Spiderman movie.
Due to Inton, Madara is able to control that jutsu(probably) better and it's also more difficult to stop it.
In short: Inton Raiha= Inton+Raiton
Well, it's also possible to create a jutsu which uses at least two Seishitsuhenka like Raiton and Katon(no combination of these two elements. That would require a kekkai genkai). But that's tough stuff. Naruto did smething similar with Yōton and Fūton.
I also believe that amaterasu and black lightning have a connection to it as well
Amaterasu is-according to the databook- just Katon, the strongest form of Katon and represents the material world. That's the complete opposite of "In".
Black lightning- only achieved by Darui, Third raikage, CM of heaven and Hagoromo's Yin chakra (Not sure, but the tattoo on Darui's shoulder could be a seal.
It did say Third Raikage passed his lightning down to Darui and only Third and Darui have that tattoo so if nothing else its possible)
Actually, if that would be true, then the name of Daruis jutsu would also change, but that's not the case. The black lightning is an advanced form of Raiton, but there isn't a second Seishitsuhenka involved.