  We investigate experimentally and theoretically the interference at avoided
crossings which are repeatedly traversed as a consequence of an applied ac
field. Our model system is a charge qubit in a serial double quantum dot
connected to two leads. Our focus lies on effects caused by simultaneous
driving with two different frequencies. We work out how the commensurability of
the driving frequencies affects the symmetry of the interference patterns both
in real space and in Fourier space. For commensurable frequencies, the symmetry
depends sensitively on the relative phase between the two modes, whereas for
incommensurable frequencies the symmetry of monochromatic driving is always
recovered.
