I got this idea from reading this guide, by Dagr: https://test-steamproxy.haloskins.io/sharedfiles/filedetails/?id=394682863

First you will need a controller emulation software I just used VJoy.

https://www.headsoft.com.au/?page_id=76

and download VJoy Virtual Joystick Driver v1.2 (Includes key to joy application) found at the bottom of the page.

Then install the program and open it.
The program will pop up in the system tray.


(Image from https://www.headsoft.com.au/?page_id=76)


Set it up so that you have Button 1 assigned to any key you want (Ideally with keys that won't interfere with you starting or playing the game).
Here's the set up I used

Click Ok

Then you can start multiplayer, you hit enter with your keyboard, and press the key assigned to button 1 (in this case it’s 1 for me). This will pop up two players. Now you can a game.
You don’t really need to do anything for player 2 (Maybe player 1 as well though I just played through the song) you can just let it idle.

You will get the achievement once the song ends.