I think they’re both at fault for that.
Player Koichi and Player Michael both seem to be pretty equally matched so far.
Both of the people involved in the car accident were talking on their cell phones at the time of the crash.
When you have both someones, you have both people. You'll notice over time that 者 is often used to indicate a person or type of person, so keep that in mind for future words as well!
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.