public class A
{
    public List<B> Bs { get; private set;}

    public void AddB(B b)
    {
        Bs.Add(b);
    }

    public void RemoveB(B b)
    {
        Bs.Remove(b);
    }

    public int DoWork()
    {
        // Need details of B
        // Probably need to inject a service that gets B details...
    }
}

public class B
{
    public A A1 { get; private set; }
    public A A2 { get; private set; }

    public B(A a1, A a2)
    {
        A1 = a1;
        A2 = a2;
    }

    public void ChangeA1(A a1)
    {
        A1.RemoveB(this);
        A1 = a1;
        A1.AddB(this);
    }

    public void ChangeA2(A a2)
    {
        A2.RemoveB(this);
        A2 = a2;
        A2.AddB(this);
    }
}