Running Two TT into one phono input, best options?

this is an example of a ’half lifted’ signal switching box. It is not preferred for phono switching. All inputs and outputs share their ground noise as a signal injection. Fairly unnoticeable for line level signals but probably noticeable for phono signal levels.

In proper phono use, the signal is balanced and the negative floats and the ground is separate and involves shielding as a concept and execution.

Some cartridges ground the negative, which is..not perfect, as concepts and execution goes. Many a cheap MM is done this way. Just to avoid hum issues in common cheap design, when the given cart is applied in a given system.

Again, the minimum of a four point switch and possibly a 5 point switch. Four and six point switches are widely available, five would be rare. Buy the six and don't use the one extra, easy-peasy. Goldpoint makes these, as do others.

Then the switch box itself, sheer size and mass can get into ground noise and ground noise injection problems.

Uber, If you are hoping for someone to tell you there is a perfectly transparent switch to solve your problem, you are hoping in vain.  I do know that Dave Slagle of EMIA can make you a fantastically good switch to solve your problem, but even he would probably not want to claim it's going to be perfect.  Using two Y adapters is absolutely the worst idea, IMO, because that does crazy things to the impedances seen by both the two cartidges at one end and the phono stage at the other end.  I wouldn't go with something cheap in hopes it will sound great.  When you come right down to it, plugging and unplugging ain't so bad.

Lewm
I do not expect there to be a perfect solution I agree but was hoping for some opinions that might be workable.

So far I have three considerations.

1/ The Decware switch with low level tonearm inputs. Would like to hear a reply to my question to them about what they changed first but it does have a 30 day money back guarantee.
2/ A Denon AU-320 SUT. This is a unit I have owned and used in the past with great results, it has 2 inputs, 1 output and is switchable from 3ohm to 40ohm to straight pass through. I honestly did not notice any readily audible difference when I last used one.
3/ A fairly cheap MM only phono stage to run the Rega mono table into and remove one of the tape decks from my system thereby freeing up a single ended input. Then both the 401 and the Avid could run into the Goldnote PH-10 phono.

I also went thru this recently, started a thread, found a few options, members added a lot to the discussion and suggested choices

https://forum.audiogon.com/discussions/phono-eq-small-inexpensive-two-phono-inputs

For MM, any simple RCA a/b switch will do it, I had an old small Sony Video Switcher, a,b,c,d in, 1 out. Best arm/cartridge direct to phono 1, a/b switch to phono 2 for the other 2 arms.

Then I decided to enter the world of Moving Coil

Any Step Up Transformer (SUT) with a combination of 2 selectible inputs, and PASS will do it, that lets you keep using your existing preamp’s phono, 1 direct, 2 thru SUT.

I got this, 3 selectable inputs. I love it, versatile options for MC, and PASS for MM, separate selector for the 3 rear inputs. You could run all 3 arms thru this, then into your Phono 1 input, use any combo of MM/MC, 

https://www.ebay.com/itm/Fidelity-research-FR-FRT-4-Toroidal-Transformer-In-Excellent-Condition/333648114135?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Luxman made switches under their top class turntable as as option to connect phono cables from two tonearm to one little box (located under the turntable). It was made in the golden age of analog for use with phono cartridges of that era.

A separate Luxman Line Selector Box is very easy to upgrade with modern high-end components (and a bit of soldering skills). I've seen this box in use with high-end tonearms, can't remember, i think it was on youtube. I believe this box is a good quality. 

The best solution is a phono stage with as many phono inputs as you need.