In the Kojiki they are called Izanagi and Izanami - but in the Nihongi (published just shortly after) they are called the gods of "in" and "yo", which seems fairly explicit to me.
It's also interesting to look at Ueshiba's rewriting of "kami" to "fire" and "water".
It is also incredibly relevant to understand that it is in the midst of these that we find power. And very few martial art teachers know how to do it. It is evident in their movement.
I mentioned a quote here from a personal discussion and push hands testing of a powerful ICMA teacher who arrived in Japan and taught Aikido and Daito ryu people. His comment about ki was interesting. He asked some heavy hitters in Japan; "Where is Yin (In)? Where is yang (yo)? Where then is this Ai-ki. You cannot pretend dantian. You will be found out."
His experiences echo my own. What does it mean when people "extend ki" or use "aik-ki" and yet you can knock em all over the place at will. They asked Ueshiba this. "Very simple"
he said "You do not understand in and yo"
Standing in the midst is a state where the mind gives out first in your training. You can have people stand in a room and have their bodies heat up to the point that you feel it in the room. This can be done in one way without specific breath patterning, and in another with breath patterning. This was supposed to be the state in various forms of Solo training, and in arts like yoga (which now substitutes EXTERNAL heat sources for what used to be internal in some systems) and was clearly demonstrated by Wang Chushin when his students would place their hands on his body in the winter to warm their fingers.