Thomas Unterberg and Robert Towbin, who resigned their posts at investment banker L F Rothschild, Unterberg, Towbin a week before Christmas (CI No 586), have accepted senior posts at the Shearson Lehman Brothers Inc arm of American Express. The team will jointly head up Shearson’s domestic and international technology group. They will also form a triumvirate with managing director Fred Frank to manage Shearson’s venture capital investments. They left Rothschild after disagreeing over a plan to change direction and reduce the firm’s stress on bringing high-tech new issues to market.