Ill try but im not very technical and time is short the next 2weeks.
Currently into hour 13 of a 8 hour day 
i was thinking of how currently retro_device_xxx behaves now, wherein mouse/lightgun gives you value depending of how much you move the mouse, and pointer gives you coordinate based on -0x7fff to 0x7fff min-max for x/y coordinates.
Would someone be able to drop the quick steps in for setting up and testing dual mouse for PC
Hopefully it will help me and others get the tested
Configuring RetroArch for multi-mouse use
Input Drivers: As of September 2017, there are two RetroArch input drivers with support for multi-mouse: udev and the Windows raw driver. It’s possible that other drivers will support multi-mouse in the future, although for some input drivers it is inherently impossible due to their design.
Emulator core and game: The reference emulator and content for multi-mouse is MAME 2003 with Area 51 (area51.zip and its corresponding CHD). If possible, test with this ‘known good’ combination before trying others.
Determining mice index numbers
- If possible, start RetroArch from the commandline with the options
--verbose --menuso that you can see status messages. (If that’s not practical, folks can use trial and error to find the indexes.) - Make sure that your input driver is set to one with multi-mouse support. If you change this setting, quit RetroArch and restart
- Load Area 51 with MAME 2003. We are doing this only to learn what index numbers have been assigned to the attached mice. If you are using udev, there should be a section of the log similar to this.
[INFO] [udev] Adding device /dev/input/event11 as type ID_INPUT_KEYBOARD.
[INFO] [udev] Adding device /dev/input/event3 as type ID_INPUT_KEYBOARD.
[INFO] [udev] Adding device /dev/input/event12 as type ID_INPUT_MOUSE.
[INFO] [udev] Adding device /dev/input/js0 as type ID_INPUT_MOUSE.
[INFO] [udev] Adding device /dev/input/mouse1 as type ID_INPUT_MOUSE.
[INFO] [udev] Adding device /dev/input/event4 as type ID_INPUT_MOUSE.
[INFO] [udev] Adding device /dev/input/mouse0 as type ID_INPUT_MOUSE.
If you are using Windows raw, you are looking for something like this:
[INFO] [WINRAW]: Initializing input driver ...
[INFO] [WINRAW]: Mouse #0 \\?\Root#RDP_MOU#0000#{378de44c-56ef-11d1-bc8c-00a0c91
405dd}.
[INFO] [WINRAW]: Mouse #1 \\?\ACPI#IBM3780#4&2b110597&0#{378de44c-56ef-11d1-bc8c
-00a0c91405dd}.
- For each user and mouse you wish to configure, visit the corresponding user ‘binds’ screen and set
Mouse Indexto match the indexes discovered earlier.
Note that my laptop registers two mice with Windows raw even though there is only a single trackpoint. However with this as a starting point you can now configure the individual player inputs even if there is a final bit of trial and error.
Wow, that looks comprehensive. I hope to get some time tonight so this is working out good.
Well not a good start im afraid. (RA 1.6.7 nightly 24/9/17) updated mame2003 core
Couldnt find a working area51 chd/zip combo so had to use area51mxr3k.
Managed to run --verbose and see my mouse#0 1 & 2 HID#VID values in the log output. (Trackpad and 2 usb BT mouse)
With the driver set to raw i get no responce from any mouse with any index combination.
When set to dinput the screen flickers on mouse click but the crosshair is very random and jumps around on click. Invisible on movement. Which is a little odd becauseat 1st run before changing anything the crosshair would follow the mouse, all be it not closely
Update i had a rouge game opt file overriding mouse to pointer. Lol
All working on my laptop.
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Nice! I hope more and more people start to take advantage of this feature
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Its really cool. I dont think theres anything more fun than 5mins of dual lightguns with a mate.
I really hope the feature gets polished off over time. If only it makes it into current mame core
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i managed to get fceumm to support dual-mouse inputs as seen in pic with player1/player 2 scoring:
:
but i dont understand how both players can play at the same time since mouse moves the same cursor and the core only has one crosshair.
With my testing at 1st both mouse controlled one crosshair. Maybe its not actually registing 2 mouse devices. But the fact it has both players scores is odd.
Have you found a video of the original game to compare gameplay to?
which core are you testing?
Sorry just the mame2003 core
i am only able to move one crosshain in area51, maybe this multimouse thing does not work well with udev
Update: Ok im using Windows now and area 51 still does not respond to 2nd mouse. i use fceumm to confirm that both mouse have its own port(normal nes uses main mouse-which zapper is connected to player2, VSuni roms using 2nd mouse, which is connected to player 1) But area 51, only the 2nd pointer moves.
[INFO] [WINRAW]: Mouse #0 \\?\HID#VID_24AE&PID_2001&MI_01&Col01#8&3e391eb&0&0000#{378de44c-56ef-11d1-bc8c-00a0c91405dd}.
[INFO] [WINRAW]: Mouse #1 \\?\HID#VID_09DA&PID_90A0&MI_01#8&6a2bfd7&0&0000#{378de44c-56ef-11d1-bc8c-00a0c91405dd}.
Update: controls are wrong here. its moving the other crosshair while its shooting on the crosshair that is not moving.
With 88 posts in this thread, I think it would be good to have a separate thread as a guide for configuring multi-mouse, separate from the ongoing development/testing conversation.
I’m hoping that more people will begin using this feature soon, and that it will be useful to have some standard instructions available for reference: Configuring RA for multi-mouse (spinners, lightguns, tackballs)
To whom it may concern: casdevel completed an overhaul of the udev input driver including support for absolute mouse devices like the Dolphinbar/Wiimote. Grab the latest version from git or the buildbot to give it a try.
This means that Linux users, as well as (some?) Android users and OS X users should be able to use lightguns including multiple simultaneous lightguns.
Just a quick test update. Tested using a Wiimote+DolphinBar again on various NES and SNES cores in WIndows 10. Cursor sensitivity is still much too high to be playable on every core I tested except FCEumm, which works well if Zapper Mode is set to pointer.
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How far are you from the Dolphinbar when testing?
About 6-7 feet, I think it only got wacky when you were really close, at least on the actual Wii.
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