One turntable with two arms, or two turntables with one each - which would you prefer?

I am not in anyway meaning to denigrate Panzerholz, but the prior discussion centered in many cases on "hardness" not just of Panzerholz but of other materials mentioned. And I wondered why. Seems to me that hardness is concomitant with the necessary characteristics of a good plinth material but is not THE reason why a material is good or bad for a plinth. Yes, a plinth ought to be good at dissipating energy put into it by the turntable chassis. By the same token, granite ought to be good too, but many (not including Pindac) have found it to be less good than other choices. Until now, I did not realize that Pindac had a favorable opinion of granite. For that matter, slate might seem to be a good choice, and it is what I chose back when slate was all the rage and OMA were selling slate plinths for a wide variety of turntables. (Given my contrarian nature, I created my plinths for the DP80, SP10 MK3, and Lenco "from scratch" but using professionals to do it the way I wanted.  When I wrote about my slate adventure on line, OMA were not happy with me.) I am satisfied with the outcome, but I am not going to claim slate (Pennsylvania black variety) is the best choice. Around that time, Albert Porter was selling Panzerholz plinths for SP10s Mk2 and Mk3. I’m sure those are excellent too. It’s interesting to me that Panzerholz is good because it’s made by glue-ing together layers of material. Glue creates a boundary between layers. Energy arriving at the glue line would be partially reflected back and partially transferred across the boundary. I am guessing that because the whole is compressed under very high pressure, that potential issue is ameliorated.  Another choice is concrete; I have used large square pavers from Home Depot as audio shelving.  It works pretty well for that.  Would be hard to conceive of making a plinth of it but fun to try.

Why do we seem to think “hardness” is the most important criterion for a material used to build a plinth? 

I have 3 turntables mounted in slate plinths, using Pennsylvania black slate. (I assume not all slate from all sources is created equal.)  The quarry in PA that sold me the slabs also was able to hone both sides of each slab perfectly flat and champfer the edges. Then in York, PA, I found a waterjet company that "learned" to cut my 3 slate slabs so to fit respectively, my Denon DP80 (50mm thick), my Lenco with PTP top plate (50mm thick), and my SP10 Mk3 (75mm thick).  In the case of the Mk3, the slate alone weighs about 75lbs, after cutting out the hole for the chassis.  I had a subplinth made out of solid cherrywood, for the Mk3 only, and bolted that to the bottom side of the slate slab.  I tell myself I can hear a tick better performance due to combining slate with cherrywood. This all sounds very good to me, but once having made the effort, I would not do it again.  Nor do I fret about whether or not Panzerholz or some other material might sound better.  Suffice to say I think all these materials are excellent choices.  Slate is just the road I went down.  I also agree, granite not a good choice; I don't really know what it is about the internal structure of granite that makes it less suitable than these other materials.

Dogberry, what phono stage provides 5 pairs of phono inputs?

I have 5 turntables with 5 tonearms up and running into two systems.  I think that makes me a bit crazy, but I viewed it as a project that required the equipment so I could make judgments for myself.  Also, I hate selling anything.