Why SetCallbacks release former bindings?

Version: Input System Preview 4

In InputAsset's auto generate codes,
I found that SetCallbacks will release the previously bound interface before binding the new interface,
This makes it impossible to bind more than one object with it .
Hope someone can tell me why and how to bind more than one object.

Code (CSharp):

1. public void SetCallbacks(IGameplayActions instance)
2.         {
3.             if (m_Wrapper.m_GameplayActionsCallbackInterface != null)
4.             {
5.                 @MoveHorizontal.started -= m_Wrapper.m_GameplayActionsCallbackInterface.OnMoveHorizontal;
6.                 @MoveHorizontal.performed -= m_Wrapper.m_GameplayActionsCallbackInterface.OnMoveHorizontal;
7.                 @MoveHorizontal.canceled -= m_Wrapper.m_GameplayActionsCallbackInterface.OnMoveHorizontal;
8.                 @MoveVertical.started -= m_Wrapper.m_GameplayActionsCallbackInterface.OnMoveVertical;
9.                 @MoveVertical.performed -= m_Wrapper.m_GameplayActionsCallbackInterface.OnMoveVertical;
10.                 @MoveVertical.canceled -= m_Wrapper.m_GameplayActionsCallbackInterface.OnMoveVertical;
11.             }
12.             m_Wrapper.m_GameplayActionsCallbackInterface = instance;
13.             if (instance != null)
14.             {
15.                 @MoveHorizontal.started += instance.OnMoveHorizontal;
16.                 @MoveHorizontal.performed += instance.OnMoveHorizontal;
17.                 @MoveHorizontal.canceled += instance.OnMoveHorizontal;
18.                 @MoveVertical.started += instance.OnMoveVertical;
19.                 @MoveVertical.performed += instance.OnMoveVertical;
20.                 @MoveVertical.canceled += instance.OnMoveVertical;
21.             }
22.         }