 As a matter of fact, one can show that the standard formalism of quantum mechanics does not allow such simultaneous measurements. But this is not a consequence of relation  (9).  Rather, it follows from the fact that this formalism simply does not contain any observable that would accomplish such a task. The extension of this formalism that allows observables to be represented by positive-operator-valued measures or POVM’s, does allow the formal introduction of observables describing joint measurements (see also  section 6.1).  But even here, for the case of position and momentum, one finds that such measurements have to be “unsharp”, which entails that they cannot be regarded as simultaneous accurate measurements.