How to get stereo and mono signals simultaneously?
Instead of just shorting the channels, you should be mixing them with two high-value resistors, one in series of each channel

L ----+-------&gt; to stereo amp
      |
     | | high-value resistor
      |
      + ----&gt; to mono amp
      |
     | | high-value resistor
      |
R ----+-------&gt; to stereo amp


The idea is super simple: Amplifiers have an input impedance that is typically much higher than what the sound source drives, so that it really doesn't matter to them that there's series resistors in the way. However, these high-value resistors will significantly reduce the amount of cross-talk, unlike shorting the channels.

If the cross-talk is still too high, you'd just place opamp voltage followers between the resistors and the stereo channels. These basically have infinite isolation.