(B) they shall jointly manage and control the assets of the plan, except that nothing in this subparagraph (B) shall preclude any agreement, authorized by the trust instrument, allocating specific responsibilities, obligations, or duties among trustees, in which event a trustee to whom certain responsibilities, obligations, or duties have not been allocated shall not be liable by reason of this subparagraph (B) either individually or as a trustee for any loss resulting to the plan arising from the acts or omissions on the part of another trustee to whom such responsibilities, obligations, or duties have been allocated.