Why Time division multiplexing lets several independent signals share one channel by taking turns: each signal gets its own repeating time slot, and the receiver reassembles the streams by keeping sync with that slot timing. Because only one stream occupies the channel at any instant, no extra frequency space is needed, just fast enough sampling and switching. D-STAR voice plus data and TDMA cellular systems work this way.
Watch out Assigning data streams to discrete sub-carriers describes frequency division multiplexing (as in OFDM), where the streams are separated in frequency rather than in time; quadrature modulation is a modulation scheme, not a multiplexing method.
Time division = take turns in time slots; frequency division = each stream gets its own carrier.