Most electrical knives will be aggressive towards each other..
The few species I know of that will live peacefully togethor are Steatygons Elegans, The 'Centipede' Knife,
and Eigenmannia virescens, the 'Glass' Knifefish, 'Electric' Knifefish, 'Green' Knifefish..
Also most the African knifefish species, but not the Aba Aba!
I will start a knifefish photo thread for ya'll

Stay tuned!