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Demystifier said:it's trivial to make an appropriate generalization. Instead of eigenstates |k⟩ with eigenvalues k one can introduce eigenstates |k,l⟩ with eigenvalues k, and in subsequent equations replace label k with label k,l when appropriate.
A. Neumaier said:No, because the decomposition is no longer unique. Thus your subsequent argument depends on which decomposition you choose.
In the case of a spin, ##l## is the position of the measured particle, hence a continuous index. Therefore (3) cannot be valid after your suggested change. Otherwise it would be valid in the limit where the position parts of ##k_1## and ##k_2## tend to each other, which contradicts (4).Demystifier said:The final result does not depend on it. It is easy to show it, so I leave it as an exercise for you.
Thus the proposed exercise is ill-conceived.