Stereo/Mono switch via switching jacks

[swinginguitar]

Does anyone have a sample schematic for a circuit that will accept independent input left/right if both jacks are plugged into, but splits the left signal to the L/R return path if only the left jack is inserted?

would passive splitting be OK or would it be better to buffer it?

does such a jack exist to do this with a TRS "stereo" jack that also switches?

edit: if it matters, this would be for an input...as in an FX return...in other words, if stereo is returned, separate signals are sent  to the rest of the circuit. if only left is inserted, it is split to the rest of the circuit

[ashcat_lt]

Use an NC switched jack.  Connect the switching contact from the right tip to the actual tip contact on the left.  It really is just that easy.