RCA cable splitter

Most turntable outputs require a higher sensitivity input on the receiver than other sources, and the input impedance is important. Unless your turntable has a built in phono preamp, you will need to use an input specifically labelled PHONO. Your computer does not have this typically. If you use an outboard phono preamp like the link below you could then split the output to drive two devices.

http://www.parts-express.com/behringer-pp400-microphono-ultra-compact-phono-preamp--248-6348

A turntable output does not have any bass and the signal needs to go through a turntable preamp to be usable as an audio source

This is the turntable I purchased: http://www.audio-technica.com/cms/turntables/583f30b3a8662772/ It has a switch to go from stereo to PC, but only one RCA cable. Thanks!

For PC play the USB should be all that is needed and the unit seems to have a phono-to-line amp built in. Set the apropriate switch on the unit to "Line" and you shouldn't need anything else. I would expect both outputs (USB and Line) to be active but that may not be the case. See page 6 of the manual.