Need to inherit from EventArgs AND ScriptableObject

This is not possible.

Can't you simply create another class inheriting from EventArgs with a simple getter&setter to access the instance of the Scriptable Object?

Create two classes :

1. One inheriting from ScriptableObject with the CreateAssetMenu attribute, and all the data you need

2. Another one, inheriting from EventArgs containing a simple reference to an instance of the class declared above

 [CreateAssetMenu()]
 public class MySO : ScriptableObject
 {
     // ... your data
 }
 
 public class MySOEventArgs : EventArgs
 {
     public MySO SO { get ; private set ;}
     public MySOEventArgs( MySO so ) { SO = so ; }
 
 }
 
 // .. Somewhere in your code:

 public event EventHandler<MySOEventArgs> SomethingHappened;
 public void RaiseEvent( MySO mySOInstance )
 {
     MySOEventArgs args = new MySOEventArgs( mySOInstance );
     if( SomethingHappened != null )
         SomethingHappened( this, args ) ;
 }