My journey last year, lots of help here, led to an extremely happy result, one deck with 3 tonearms: Long 12.5" Arm for MC; 9" Arm for MM; Compact 9" Arm for MONO. All aligned, balanced, instant back and forth Stereo/Mono in a listening session. I play a lot of Jazz, many Mono LPs.
It began with a Vintage JVC Dual Arm Plinth CL-P2: Stereo Arm and Mono Arm.
This, from Japan is expensive shipping, but shows the JVC deck well:
https://www.ebay.com/itm/Victor-JVC-CL-P2D-Plinth-for-TT-71-TT-81-TT101-Direct-Drive-Turntable/32440... Then I realized I was using my MC to play LP’s that were fun but not audibly dynamic, or to test LP’s for sale, wearing out a non-replaceable Stylus, so I squeezed a 3rd arm on the left side, to get the 3 types, use the replaceable stylus for ’less than special’ LP’s
My finished 3 arm deck can be seen in the last photo of my eBay listing:
https://www.ebay.com/itm/133612076659New 3 arm solutions exist, the special 7 layer plinth is terrific.
I went with TT81. None currently. Check out previously sold items on hifishark:
https://www.hifishark.com/search?q=victor+tt-81Happily I found this Vintage SUT with 3 inputs, bypass for MM and 4 optional loads for MC, so I only need 1 MM phono input.
Now, 1 beat up, 1 too expensive, they frequently come up in eBay, and other SUT's exist with 3 inputs.
https://www.hifishark.com/search?q=fidelity+research+frt-4
Good luck with your search, expect lots of good help here.