2 turntables and one phono stage - Can I???

As Sidssp inferred, you COULD use a splitter, if you also used a switch, so that the phono circuit was only "seeing" one of the two cartridges at any one time, or to put the same concept in another way, so that the cartridge you are listening to will not "see" the other cartridge in parallel with the phono input impedance. This probably would require an outboard box containing a switch plus two pairs of phono inputs. If you decide to do that, just hard wire the from the internal leads to the one phono input on your preamp (i.e., I would advise you to avoid using yet another pair of ICs with another set of RCAs to do this).

FWIW, with respect to Almarg's comment, you should probably use a DPDT switch, so as to switch both sides of the two cartridges simultaneously. There should be no problem with that. The shield would be separated from the so-called "hot" and "ground" outputs from each cartridge. The cartridge ground should not contact chassis ground at all.

Almarg, I was in a way responding to the (good) point that you made. To do this properly, you need to isolate the lead carrying the "ground" side of the cartridge output (which as you correctly state is merely the other side of the coil from hot) from both chassis ground and the shield around the phono cable. Chassis and shield leads can be connected to the chassis of the switch box and hence back to the preamp. Then switch BOTH sides of the coils of both cartridges, with no ground reference, using a DPDT. It occurs to me that you would need two DPDT switches, one for each channel, or one 4PDT switch.