This was also possible with the Amiga version.

Yup, and I have played it with others.

My guess is that it came from the Amiga and the dos port(?) had to do some pretty unusual magic to make that work on PC?

There were mice for the serial port, for PCs. So only lesser magic. (One "standard" mouse, like "bus mouse" or PS/2 mouse, and one serial, would have worked fine.)

If one mouse goes into the keyboard PS/2 keyboard connector (I think they were the same connector) then yes, very unusual magic.

This feature was invented on the Amiga for Lemmings. The Amiga had two joystick/mouse ports fully supported by the OS so it was a natural development.

MS-DOS made it much more difficult... using one mouse required that a mouse driver be resident in the 640k base memory and connected via COM or PS/2. Settlers had internal code to access a second serial based mouse but it wasn't straight-forward at all.

The DOS port of Lemmings didn't bother.

The mouse interface on the Amiga was remarkably and elegantly simple too, which would have made it easy to interpret the mouse inputs on a joystick port even if there wasn't built-in support for it.

Basically, the mouse buttons were wired directly to pins - the same pins that joystick buttons would be wired to. Then also, the left/right and up/down steps would be indicated by a rising pulse on one pin with a direction indicated on another pin. The same pins that would be used for the left/right and up/down microswitches on a joystick. The way this was implemented by most early mice was to have two sensors on a sense wheel slightly out of phase, so as the sense wheel rotated it would produce a square wave on both wires. When one pin had a rising pulse, the other wire would be high if the wheel was going in one direction, and low if it was going in the other direction.

This is called a rotary encoder [1] and is still common in various control knobs on electronic devices.

[1]: https://en.m.wikipedia.org/wiki/Rotary_encoder

There haven't really been more double mouse games, have there? Sounds like it could have potential, if people got used to it.

Two joystick ports was common on home computers years before the Amiga.

Indeed. My Commodore 64 nearly choked on its coffee just now. There was an entire 8-bit generation that followed this model.

In fact the 8520 CIA chips used to control the joystick ports on the Amiga were compatible enough you could put them in a C64 in place.of one or both 6526's... (You'd lose the realtime clock, as the 8520 had timers instead, but "nobody" used the realtime clock on the C64).

I tested that at one point when one of the 6526's on my C64 broke. I don't think the reverse would work, as I think AmigaOS used the timers, but never tested.

(My parents were not amused that Is open the machines and swap chips around...)

The Atari 400 and 800 computers had four joystick ports. Two paddles per port meant up to 8 paddles. The only game I know used four joysticks was M.U.L.E.

I've been playing serf city with two mouses (some work was needed to set drivers right under dos) and mouse and joystick. it was awesome.