If a trust provides for one or more successor beneficiaries after the death of the settlor, the trust is not invalid, merged, or terminated in either of the following circumstances:
(a) Where there is one settlor who is the sole trustee and the sole beneficiary during the settlor’s lifetime.
(b) Where there are two or more settlors, one or more of whom are trustees, and the beneficial interest in the trust is in one or more of the settlors during the lifetime of the settlors.