Who decided there are two divisions of 12 hours in any day, and when was that decided?

The Egyptians are generally given credit for the 12-hour day and night though the division was also observed in ancient Mesopotamia. The day was determined by a sundial and the night was determined by a water clock.

https://en.wikipedia.org/wiki/12-hour_clock

Originally, the ancient Greeks divided the time of light into nine hours, then ten (from sunrise to sunset) and finally twelve (including dawn and dusk as the first and last hour). Nights were divided into four vigils, with the 12-set of dark hours being a later development.

It should be noted that, at these times - and also in the Bible - hours were not of equal length. The bright time of day was 12 hours and so was the dark time. In winter, dark hours were longer than bright ones and summer had it vice versa.

Jesus is described to have died at the ninth hour, which (given it was nearly equinox) places his death around 3 in the afternoon. Today, the Roman Catholic prayer of the hours still uses this count - prime was originally the hour of sunrise (even though the name implies 1, it is really the zero count) with terce, sext and nones coming in three-hour intervals.

https://en.wikipedia.org/wiki/Horae
https://en.wikipedia.org/wiki/Hour