You might read up on telecom multiplex. This was introduced in the 30s, using RF modulator techniques, and some very well constructed filters, to take a single telephone line (running between exchanges) that might be 100kHz useful bandwidth, and deliver 30 or more phone conversations simultaneously along it. And with electronic switching (or at least, as an option -- instead of electromechanical exchanges).
This was a basic enabling technology for transatlantic phone cables -- a single line for a single conversation would've been preposterously expensive.
The basic breakdown is this: each phone line is filtered to its exact specified bandwidth (typically 500-3000Hz, giving that classic muffled telephone sound quality; the skirts were quite steep, something like >60dB attenuation at 4kHz), and using balanced mixers and precision oscillators, the channel is shifted up or down to its assigned carrier frequency. Then that is filtered (to remove the unneeded sideband), and finally all the signals are mixed together for transmission. On the receiving end, the opposite happens: a channel is selected with a filter, and mixed down to baseband (the baseband filter only needs to remove the carrier pilot tone, frequency sum, and harmonics, which saves a little).
Note that the phone line is full duplex on both ends, so a precision hybrid is required to separate rx/tx waves. And for that precision to matter, the line itself also has to be trimmed to very flat impedance and frequency characteristics (some guy at Bell, with the name of Zobel, proved very good at this).
Tim