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

I agree with Sid and Lew that each cartridge must absolutely not be allowed to be loaded by the other cartridge. I don't know if the Dynavector DV20X listed in your system description is the high output or low output version, but in each case you'll notice in its specs that the recommended MINIMUM load impedance is around 6 times greater than the cartridge's dc resistance. And besides that, while inductance appears to be unspecified, it too is likely to be significant and would make the loading presented by one cartridge to the other (if they were connected together) frequency dependent.

I'm not sure that a mechanical switch would work very well either. Each channel of a cartridge output should be thought of as a differential, balanced pair of signals, one side of which should be grounded when and only when it reaches the rca input on the phono preamp. A typical mechanical switch will switch one of the two leads of each channel of each cartridge, but will permanently connect the other lead (the one that is on the shield of the turntable's interconnect cable) to the metal enclosure of the switch (through the rca shell), and to the shields of the leads from the other turntable and the other channel, as well as to the shields of the output cables. Which, considering the low signal levels that are involved, would seem to be an invitation to noise, hum, and ground loop problems.

Regards,
-- Al

Just buy a second phono cable & twist tie it/both near the rear of the pre amps phono section.
Works for me.
You still may have to make changes @ the pre amps phono section to match the new load.

There are phono stages that have 2 inputs...

Why don't you call Joule Electra and ask if they can add a second input? I know they offer this feature for the OPS-2 for a very modest cost. I imagine they could do it for the OPS-1 too.

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.