In the Cypher system, if two player characters are taking different actions that help each other, both get a bonus.
In the book, it's explained that in order to do that the second character has to have the skill / ability that the first is using. But, does the first character also have to have the skill / ability that the second character is using?
For example, if A uses Pierce and B uses Bash, do both characters need to have the two abilities in order to get the complementary bonus?
No, it is not necessary.
From Numenera core, p.101. The rules boil down to: Character B must know the ability A uses, and also must use a different ability.
Specifically, in the description and in both examples given (1 combat and 1 non-combat), there is no description of a requirement that A know B's skill, and thus it is not necessary.
The implication is that B is primarily responsible for weaving their action in, following A's lead. The first character announces their action (eg, Pierce, in the question's example). The second character, being familiar with Pierce, can predict the outcome/ reaction to it, and then take advantage of that knowledge to give them both a bonus (eg, knowing Pierce, and then attacking with Bash).
Of course if they do both know the same abilities, it will simply provide further opportunities to combine their efforts at other times.
