Are there "multi-frequency" signals within a human brain?
Remembering that neurons don't generate sinusoids, just spikes (mostly)....

It depends on exactly how you want to define "depend on".  For instance, a downstream neuron receiving inputs from two neurons will be more likely to fire as it spatially/temporally sums the inputs from the two neurons.  Regardless of their specific rates, if their interaction is such that an impulse from each neuron arrives within a certain timing of the other, it may be sufficient to cause the downstream neuron to spike.  In this case, particular sets of frequencies of the two upstream neurons would be more likely to activate the downstream neuron and would do so at a particular rate for the downstream neuron.  Of course, the phase delay between the two neurons would be important too.

In a more general sense, one might consider the binding problem, by which many sensory inputs are bound into a single, higher level experience, such as an object or a biographical memory, which can then later be recalled (and thus activate all the lower sensory regions that encoded the memory in the first place, nearly recreating the experience).