Interesting topic. Just to put in my $s worth, I would not be comfortable & would never suggest combing stereo cartridge outputs as you will then be loading each channel with the other. This would then mean that each channel is "looking into the other" & creating a serious loading problem.
Even when combing channels (Y cord etc), you have to make sure of this loading problem as well. Outputs from the various stages have a very low source impedance whereas inputs are high impedance -10K or so. Hence when you combine outputs you need build-out resistors to prevent undue loading from one channel "looking into the other". Shorting or combining inputs is OK though. So be careful with Y adapters.
Even when combing channels (Y cord etc), you have to make sure of this loading problem as well. Outputs from the various stages have a very low source impedance whereas inputs are high impedance -10K or so. Hence when you combine outputs you need build-out resistors to prevent undue loading from one channel "looking into the other". Shorting or combining inputs is OK though. So be careful with Y adapters.