Using different frequencies for transmission from multiple sources or in opposite directions is called frequency-division multiplex (FDM). This implies that each transmission sources has hold of a dedicated frequency band it can use.

Nearly all Ethernet PHYs use baseband signaling - baseband means the frequency starts at (close to) 0 Hz and reaches up the maximum frequency that is used. Therefore, baseband signaling provides no room for FDM.

(Optical fiber can be considered a special case: the baseband signal modulates the amplitude of a light source - using different light colors/wavelengths you can very well multiplex various sources into a single fiber. This is used for e.g. 1000BASE-BX10 or 10G/GE/PON.)