That's an option, but you'd risk sending an input signal from one device (e.g. the turntable) into the output jack of the other device (e.g. the iPod). I can't say that it would damage either one, but it's a risk.
I'd recommend using an A/B switch box instead (either that or have the cables go into an adapter that lets you only plug one of them in at a time). Something like
this should work fine (I say "should" because I haven't seen the back, but I'm assuming from the description that it uses RCA inputs). There are a number of switch boxes out there, some with remotes, so that was just an example to get you started.