They are not two different specie, they are two different genus.
Xiphophorus variatus, Variable platyfish. Xiphophorus is the genus, variatus is the specie
Poecilia latipinna, Sailfin Molly. Poecilia is the genus, latipinna is the specie
Fish of the same genus can interbreed. Fish not of the same genus, can usually not interbreed. I say "usually" not because I believe I've read of rainbowfish of the same family breeding across genus, but I'm not 100% sure there (Melanotaenia x Glossolepis).
Roan