Initially, ensure that Rocket League is not running and your DualShock 4 is not plugged in.


Then follow these steps:

1. Start DS4Windows.
   
2. Ensure that the Hide DS4 option is turned on
   
3. Connect your DS4. (I only tried it with the USB cable.)
   
4. Start Rocket League and navigate to the main menu.
   
5. Alt+Tab out of Rocket League and turn off the Hide DS4 option in DS4Windows.
   
6. Switch back to Rocket League and press the "Options" (Start) button on the DS4. You should see another car appear on the main menu.
   
7. Alt+Tab back to DS4Windows and click the Stop button (or just exit DS4Windows from the system tray).
   
8. Switch back to Rocket League. You should be good to go.

This is mostly speculation.

With "Hide DS4" turned on and DS4Windows running, Rocket League sees a virtual Xbox controller. With just one "controller" plugged in, this controller is assigned to Player 1 (alongside the KB/M). When "Hide DS4" is turned off, Rocket League additionally sees the actual DS4, and assigns the new controller to Player 2. At this point, if you started a game, the controller would control both players at the same time. Turning off DS4Windows makes the Xbox "controller" disappear, but by this point, the DS4 is already locked to Player 2.