Consider Functions Of The Second ARM
1. make sure one of the arms has removable headshell
2. that gives you freedom to try MM, MC, MONO, ... listen to your friend's cartridges on your system.
3. above assumes you are able/quick at checking null points, recalibrating tracking force, and anti-skate when changing cartridges, and easy azimuth. Note: micro seiki 505's have easy adjustment of their arm fitting. Headshells with azimuth adjustment can be acquired.
4. 2nd arm should have EASY arm height adjustment in case cartridge height varies. My experiences: Acos Lustre GST 801; Technics 500 Base; Micro Seiki 505s. My Long Russian arm should be easy, micrometer lift, but it isn't.
5. How do you switch between arms into your phono stage?
6. How do you adjust x factors and impedances for various MC cartridges? Switch between MM and MC (SUT) I didn't think about that, and I have 3 arms.
My solution was Fidelity Research FRT-4 which has 3 inputs, front selectable, 4 front optional x factors and impedance options; and PASS for MM thru it. Thus only one set of cables into my Phono Stage.
Entre 100 is similar.
IOW, the mechanics of 2 or 3 arms needs to be solved.