usually you can short inputs together, (both inputs just receive the same signal), but not outputs. When you connect outputs together, then either output is likely to output a (slightly) different voltage, and this may have strange effects. From getting hot or instability / oscillations. For the outputs, if you want to combine them, then add series resistors to them. This will average out small differences between the two outputs.
Or just use either the left or right channel (both for input and output) and not the other.