mutual; reciprocal
Mutual respect
Reciprocal protection in wartime
Your mutual animosity comes to nothing;  your mutual animosity is of no avail
mutually;  each other;  one another
As a general rule, mutually opposed tendencies result in war
His three sisters are always envious of one another;  his three sisters always envy one another
The two girls imitate each other by wearing identical hats
To reciprocate each other's greetings;  to exchange greetings (with each other)