I am a bit confused...
Does that mean the for an Enterprise 128 + EnterMice  the best choice is 1 master + 1 slave adapter,  although 2 master adapters can also work ?
Also, if I plug a Master adapter to a C64, do I need to switch off / disable something in it ?

Entermice's joystick ports side is "only" a hardware solution.

This Sega Twister gives the Enterprise the "possibility" to manage six buttons game-pads. But the games that support all that buttons have still to be created...

On the other side, the Entermice two joystick ports are already supported by the vast majority of Enterprise games and also by Basic(EXOS).

A Sega game-pad plugged on a Sega Twister+Entermice acts as a normal two buttons joystick if the selecting code is not provided. Until now there isn't any game with the selecting code inside it, but now thanks to Pear it is a "possible possibility" only if someone implements it on a game.

Why master and slave? because the same DATA-OUT from the serial port can suit a number of Sega-Twisters. But that line outputs +12v and we need +5v, so inside the master the conversion is done. Slave ones only need +5v from a master one output.

C64 Joystick ports have already a Select(+5v) signal on pin 6 of D-sub 9, but again, its pin-out don't match the Sega pads one, and the game has to have some sort of selecting the pin.

The Neos mouse, uses that pin to read nibbles. C64s can select that pin because the Neos mouse is a MSX mouse adapted to one of the C64 joy-ports.

I don't have the slightest idea how it is done as I only had Amiga from Commodore, but that is a fact....

With the code stems that the CIA joystick ports in C64 are bidirectional.
I did not know.