Almost all analogue leads carry a single mono signal. Therefore, to connect a device in stereo you must use 2 leads, 1 each for the left and right channels. To monitor the stereo effect, you must pan them left and right. There is 1 exception ... headphone leads carry a stereo unbalanced signal down a single cable that utilises a TRS (stereo) 1/4" jack plug or stereo mini jack.
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In the sound recording studio almost all analogue leads carry a single mono signal. To connect a device in stereo you must therefore use 2 mono leads (left and right).
There is 1 exception ...
Because digital signals can encode multiple channels in a single signal only one lead is required to carry mono, stereo and multi-channel signals. Examples include ...
Note: AES (XLR connectors) and SPDIF (phono connector) leads require a different cable to their analogue counterparts (mic and phono leads).
NOTE: You may also want to read about digital audio synchronisation.