I have 3 turntables going into one preamp. They all have their own phono preamp, and those phono preamps are plugged into line level inputs on my line level preamp. One of the main problems with hooking up multiple preamps, is the fact that their ground is not also switched on and off as their inputs are. So, if you have 2 or 3 tables plugged into one phono preamp with a switcher, the grounds of the 3 turntables are still connected. The tables that are unused become antenna(s) for the one that is being used and they gather all the emf about, resulting in hum.
My old Marantz 3300 preamp has 2 phono inputs and only 1 phono preamp board inside, but man, did they ever do some major shielding. It works, but I'm sure it was at a considerable expense. They did not switch off the ground thru their system, but 2 tables was the max.
'ner