This is a simple one, but we get this question a lot. “I just bought my first multi effects processor which has a left and right input. I am not sure if I need to run two effect sends to it and plug in both inputs, or do I only use one effect send and plug in the side labeled mono?”
If your multi-effects processor is capable of two independent effects at time, and these effects can be applied independently to the two different inputs, then you should use both inputs. Also if it is a “true stereo” (see WFTD true stereo above) processor, you should use both inputs. Many processors are not “true” stereo, which means they sum the inputs to a mono signal before going into the actual processor. They have two inputs so you can keep the dry portion of your signal in its original stereo form if you are using the device in line with other units. If you are using a processor of this type in an effect send/return scenario (as you would with most mixers) then you should only connect to the mono input (which is usually labeled). If one isn’t labeled, then just pick one of the two, but there is no need for two connections to the input.
You can usually find good information about the way in which your specific model needs to be connected in the owner’s manual. The advice above is generic; there are dozens of exceptions to these basic rules that will come up from time to time.