ok. Robin said, due to the fact that both sides of the cartridge use the same mono signal; when connecting two stereo outputs it "can" cause a ground loop. this does not always happen. in fact; i've had it when it did not cause a ground loop....and i don't know why it does not always happen.
but if it does; then you get a sum'ing interconnect (where you have 2 rca inputs into one rca output) and plug that into one side of your preamp. and then use the mono switch on your preamp to get 2 channels of mono. you have then defeated the ground loop.
if you don't have a mono switch then you get a splitter (an interconenct with a single rca input and 2 rca outputs) plug the suming single output into the splitter and plug one output from the splitter into each stereo input. you now have a mono signal out of each input (just like a mono switch).
the most elegant solution is to get a friendly cable maker to make you the suming and splitter combo piece and eliminate the extra connectors. it's a simple thing to do.
but if it does; then you get a sum'ing interconnect (where you have 2 rca inputs into one rca output) and plug that into one side of your preamp. and then use the mono switch on your preamp to get 2 channels of mono. you have then defeated the ground loop.
if you don't have a mono switch then you get a splitter (an interconenct with a single rca input and 2 rca outputs) plug the suming single output into the splitter and plug one output from the splitter into each stereo input. you now have a mono signal out of each input (just like a mono switch).
the most elegant solution is to get a friendly cable maker to make you the suming and splitter combo piece and eliminate the extra connectors. it's a simple thing to do.