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Imagine for a moment you know who stole your car (some thiefs for example):
Sabemos qué personas lo hicieron.
The above sentence is the same as saying:
Sabemos quiénes lo hicieron. (Persons are imply)
In both sentences we are talking about people, so my question is, if I use this form:
Sabemos quiénes personas lo hicieron.
Is this correct and/or legal? I mean using "quiénes" despite "personas" being implied?
Quién is a interrogative/exclamative pronoun, so it must resplace a noun.
On the other hand qué can be an interrogative/exclamative pronoun (when referring to things) or and interrogative/exclamative adjective (when referring to people or things). So as we are talking about people it can only be an adjective so it has to go with a name.
You can find a complete explanation from RAE about the use of quién here and about the use of qué here.
So for example the sentence you gave:
Sabemos qué personas lo hicieron. (Qué used as an adjective: Correct)
Sabemos quiénes lo hicieron. (Quiénes used as a pronoun: Correct)
Sabemos quiénes personas lo hicieron.* (Quiénes used as an adjective: Incorrect)
Sabemos qué lo hicieron.* (Qué used as a pronoun, replacing people: Incorrect)
