Well, usually the transmitter employs some form of modulation. Well, I mean, a radar transmitter does not always transmit pulses. It transmits signals with a wideband frequency content. (You need wide bandwidth to resolve distance/time accurately. ).

Now, the important part is that the transmit spectrum has a wide bandwidth, how this is done, does not matter too much. In its simplest form, the wide bandwidth signal is a pulse. More complicated, its a chirp, a sine waveform whereof the frequency continuously rises. Some radars use sets of chirps (eg pulse-doppler), in order to resolve velocity and position at the same time.

In cases where you have multiple radars operating at the same frequency (eg automotive radar), each transmitter will modulate the time-frequency characteristic of its transmitted waveform in a unique way. (eg not just a chirp, but the frequency moving in a more complicated way), and tune its receiver so that its more sensitive to this waveform than to the ones transmitted by the other radars.

In a multistatic radar system comms techniques such as Time Division Multiplexing (TDM - http://en.wikipedia.org/wiki/Time-division_multiplexing), Frequency Division Multiplexing (FDM - http://en.wikipedia.org/wiki/Frequency-division_multiplexing) or Code Division Multiple Access (CDMA - https://www.princeton.edu/~achaney/tmve/wiki100k/docs/Code_division_multiple_access.html) can be used to allow the networked radar to operate.

What your question is specifically regarding is how can a multistatic describing is how can a receiver process the different returns if they are at different frequencies. This is possible using a wide band receiver which can therefore receive each frequency offset result and using FDM the results from each specific transmitter which is offset at a different frequency can be separated. Key issues with this are obtaining a good enough wideband receiver and avoiding spurious outputs interfering with the other frequencies transmitted.

In presence of multiple transmitters and receivers, state space model  can be developed considering frequency as one of the state variables. Then frequency estimation can be preformed using various adaptive filtering techniques like Kalman filter. But channel distortion should be considered during modeling.

There are two ways of exploiting the multi-frequency aspect of multi-static radar system:

1. Bi-static processing for each Transmit-Receiver pair.  

2. MIMO processing considering all the pairs. 

In first case, usually, target is detected separately at each Tx-Rx pair before the final decision is made using some fusion rule. 

In second case, signals from multiple frequencies are exploited to enhance detection performance. One example can be combining the bandwidth to increase the range resolution of the radar. 

Different transmitting frequencies are the more favourable condition for multistatic radar, because the transmitters may be identified by appropriate bandpath tuning at the receiver. The detection results of each transmitter/receiver pair may be combined by the subsequent tracking procedure.