ExtendEditorTest.cs

public class Hoge
{
    public int intValue1;
    public int intValue2;
}

#if UNITY_EDITOR
[CustomEditor(typeof(Hoge))]
public class HogeEditor1 : Editor
{
    private SerializedProperty intValue1;
    
    void OnEnable()
    {
        intValue1 = serializedObject.Find("intValue1");
    }
    
    public override void OnInspectorGUI()
    {
        EditorGUILayout.PropertyField(intValue1, new GUIContent("intValue1"));
    }
}

[CustomEditor(typeof(Hoge))]
public class HogeEditor2 : Editor
{
    private SerializedProperty intValue2;
    
    void OnEnable()
    {
        intValue2 = serializedObject.Find("intValue2");
    }
    
    public override void OnInspectorGUI()
    {
        EditorGUILayout.PropertyField(intValue2, new GUIContent("intValue2"));
    }
}
#endif