Wow, I love iris, but have only started collecting. I'd contact an iris society with the pictures. I'm sure it is a genetic trait, but how eyeroll obvious can my statement be?
Not genetic, at least not in the sense of inheritable.
This plant is a pretty obvious example of a chimera - two distinct tissue types growing together in the same plant, split down the middle in this case. It may or may not persist in producing flowers like this, or the proportions of the 2 colors may vary from flower to flower, or some may be single color either way. Certainly interesting to see.