I use a phono pre for each table (two, but only have one hooked up at the moment) then run those into a mixer with a decent preamp in it...
OR, you could use the phono pre, then hook it into an AUX... so, you could have one table going into your phono input, and one going into a AUX, and another could even go into another AUX if you have one. You would have to buy two phono pre's though.
I just can't imagine haveing more than one table hooked up at a time though if you can't mix them. I guess if you are just wanting a different one for a different kind of music might me one reason, but clicking a switch to turn one off and one on just wouldn't be as cool as non stop sound with a mix..