  Cloning of observables, unlike standard cloning of states, aims at copying
the information encoded in the statistics of a class of observables rather then
on quantum states themselves. In such a process the emphasis is on the quantum
operation (evolution plus measurement) necessary to retrieve the original
information. We analyze, for qubit systems, the cloning of a class generated by
two noncommuting observables, elucidating the relationship between such a
process and joint measurements. This helps in establishing an optimality
criterion for cloning of observables. We see that, even if the cloning machine
is designed to act on the whole class generated by two noncommuting
observables, the same optimal performances of a joint measurement can be
attained. Finally, the connection with state dependent cloning is enlightened.
