Even more generally, any set of orthogonal codes will do -- read up on CDMA for ideas. And it works from both sides, analog and digital -- there are
filters perfectly crafted to optimize digital signal quality.
You can dive quite deeply indeed in this field; modern radio communications takes full advantage of the immense processing power available in modern technology. (Typically, a BT, Wifi, 4G, 5G, Lora, etc. chip includes on-chip tuned circuits (filters and amplifiers), baseband or equivalent-time-sampled acquisition (ADC), DSP filtering and demodulation, code/key descrambling, error detection and correction, and some network stack. It's a magnificently complex system, all in the name of performance per dollar!)
This is probably getting rather far away from the original purpose though -- distance sensing, sounds like the intent might've been to detect the relative amplitudes of each emitter, being able to separate them by frequency or code. This isn't a great idea, because signal strength can vary from
so many factors besides sheer physical distance. Amplitude sensing is one of the least reliable methods of signal reception and detection.
A much better system is to use the relative delay between sources, and triangulate position from there. This can be tricky -- fractional nanoseconds must be resolved. But it is eminently solvable. Indeed, fractional nanoseconds are no problem with even a modest PLL and some clever circuitry (maybe something better than a garden-variety phase detector, but that's a good starting point). You might need some advanced analog circuitry or an FPGA to implement the full signal processing chain (for some level of complexity and performance), then probably a MCU to do the trig.
FWIW, distance imaging sensors
(e.g. Xbox Kinect) typically use a pulsed illumination source, and a coherent (equivalent-time-sampled) image sensor, to resolve the phase shift (per pixel!) into a signal level. The illumination source can be a modest frequency, 10s of MHz for example. Even though the phase shift is small (a few ns), as long as the detection is proportional, that's fine, it's a 2simple matter of signal level. It can also be averaged over many thousands of samples (15MHz makes 250k cycles every 60Hz frame!), giving reasonable SNR even after amplification.
Of course, a distance sensor is a monostatic radar system, different from what you're proposing.
Tim