  The addition of a weak oscillating field modifying strongly dressed spins
enhances and enriches the system quantum dynamics. Through low-order harmonic
mixing the bichromatic driving generates additional rectified static field
acting on the spin system. The secondary field allows for a fine tuning of the
atomic response and produces effects not accessible with a single dressing
field, such as a spatial triaxial anisotropy of the spin coupling constants and
acceleration of the spin dynamics. This tuning-dressed configuration introduces
an extra handle for the system full engineering for quantum control
applications. Tuning amplitude, harmonic content, spatial orientation and phase
relation are control parameters. A theoretical analysis, based on perturbative
approach, is experimentally validated by applying a bichromatic radiofrequency
field to an optically pumped Cs atomic vapour. We measure the resonance shifts
produced by tuning fields up to the third harmonic.
