Try "were mistaken." The concept [is the mistake] [of you] can be concisely rendered either:
- you made a mistake
- you were mistaken
Your first example:
A. Both of you made that mistake.
A. You both made that mistake.
A. Both of you were mistaken.
A. You were both mistaken.
To clarify even further that we are discussing two people acting together (rather than separately) on a single decision, add "together":
A. Both of you made that mistake together.
Your second example:
B. Neither of you made that mistake.
B. Neither of you was mistaken.
To clarify that this was a single collective decision, do not use neither (not either). Instead, use "you two ... not" or "both of you ... not"
B. You two didn't make that mistake.
B. You two were not mistaken.
You might also change from talking about who made the mistake and instead talk about who is responsible for or to blame for the mistake.
Neither of you was responsible for that mistake.
You two were both to blame for that mistake.