  A general formalism for joint weak measurements of a pair of complementary
observables is given. The standard process of optical three-wave mixing in a
nonlinear crystal (such as in parametric down-conversion) is suitable for such
tasks. To obtain the weak value of a variable $A$ one performs weak
measurements twice, with different initial states of the "meter" field. This
seems to be a drawback, but as a compensation we get for free the weak value of
a complementary variable $B$. The scheme is tunable and versatile: one has
access to a continuous set of possible weak measurements of pair of
observables. The scheme increases signal-to-noise ratio with respect to the
case without postselection.
