Not everyone here uses a looper with guitar. If you want to use a looper or other effect on turntables, the best way will probably be to use an effects send and then to return it to a dedicated channel. This isn't necessary, however. You can always send the mixer's output straight into the looper or effects device. This works best with stereo devices, of course. The drawback here is that if you have a different program on each deck and only want to loop or effect one of them, it's impossible. The other alternative is to use an external phono preamp and run only one turntable into the effects, and then run that into the line input on your mixer.
P.S. The Rane TTM54i has "effects send" is really an insert. Make sure that if you look into DJ mixers with effects sends, that they will work the way you want them to.