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