The Reagan Administration is conceding to Sen. William Proxmire (D-Wis.) the argument over a key point in how banks should be permitted to enter the securities business. Proxmire, chairman of the Senate Banking Committee, has insisted that any relaxation of the law separating banking and securities underwriting should require a holding company. That structure, in which a bank and a securities affiliate would both report to a parent company, is essential to preventing conflicts of interest and protecting the safety of federally insured bank deposits, Proxmire says. Treasury Undersecretary George Gould and Federal Deposit Insurance Corp. Chairman L. William Seidman would permit banks, with certain safeguards, to directly own a securities subsidiary, and vice versa. However, both men said that they would rather accept Proxmire's position than have no bill at all.