Two quartets don't necessarily make one (5)

I believe the answer is:

octet

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'two quartets' is the definition.
(I've seen this in another clue)

I can't explain the remainder of the clue.

(Other definitions for octet that I've seen before include "A group of eight instruments or voices" , "Composition for eight musicians" , "Set of eight" , "Eightsome" , "Group of eight musicians" .)

I've seen this clue in the Evening Standard.

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