Nesting custom inspectors

I have two MonoBehaviours, let's call them A and B

 public class A : MonoBehaviour
 {
     public B b;
     ...
 }

 public class B : MonoBehaviour
 {
     ...
 }

and a custom inspector for each:

 [CanEditMultipleObjects]
 [CustomEditor(typeof(A))]
 public class AEditor : Editor
 {
     private SerializedProperty bProp
     ...
     public void OnEnable()
     {
         bProp = serializedObject.FindProperty("b");
         ...
     }
     public override void OnInspectorGUI()
     {
         EditorGUILayout.PropertyField(b);
         ...
     }
 }

 [CanEditMultipleObjects]
 [CustomEditor(typeof(B))]
 public class BEditor : Editor
 {
     ...
 }

Both objects require their own custom inspectors, and both need to be multi-editable (therefore requiring the use of SerializedProperties). In addition to being nested inside of A, B can also exist on its own, and needs to be editable in both cases.

When I select an object of type B, its custom inspector shows up nicely. Unfortunately, when I select an object of type A, instead of the custom inspector for B being nicely nested inside the inspector for A, I simply get a box with a link to the instance of B contained in A.

How can I get B's custom inspector to show up nested inside of A's inspector without duplicating code?