In an extraordinary series of secret meetings, two top Reagan administration regulators have drafted a far-reaching agreement that will allow risky new investments to be sold to the public and will directly benefit the businesses the regulators formerly worked for.

Small investors will be able to speculate on whether the prime rate is going up or down or to risk their money on the rise and fall of popular stock market indicators such as the Dow Jones Industrial Average as a result of the agreement announced last week by the Securities and Exchange Commission and Commodity Futures Trading Commission.

Though such exotic investment vehicles have been denounced as little more than gambling by some congressional officials, the chairmen of the SEC and CFTC said last week they expect to approve their sale to investors sometime next year.

The decision on the new breed of investments was reached in an unusual series of six closed-door meetings between SEC Chairman John S.R. Shad and Philip McBride Johnson, chairman of the CFTC, both Reagan appointees.

The Shad-Johnson plan could bring many millions of dollars of new business to stock and commodity brokers and could directly benefit the firms that the two men worked for before joining the Reagan Administration less than a year ago.

Shad was previously a vice president and director of E.F. Hutton, a major Wall Street firm that is almost certain to sell the new investments. As a Chicago commodity lawyer, one of Johnson's biggest clients was the Chicago Board of Trade, one of the markets on which the new investments will be traded.

The decision-making process raises questions about the close ties between the two regulators and the industries they are supposed to police and challenges the principle that public-policy decisions ought to be made with public participation.

The entire decision was made with no public hearings and no opportunity for public comment. Investment industry officials, however, were told in advance about the plans of the two agencies and were consulted by both chairmen before the decision was made.

Until the talks between Shad and Johnson, the SEC and CFTC had been at odds for years over how to regulate a new generation of investments that hadn't been invented when most securities and commodity laws were enacted.

The dispute is more than just a bureaucratic turf fight, because the SEC and CFTC operate under vastly different regulatory plans. The SEC not only has tougher regulations to protect customers from abuse, but also a reputation as an effective watchdog. A congressional report on the CFTC last week charged that agency has neither the necessary rules to protect the public nor the will to enforce its own regulations.

Shad and Johnson announced shortly after they were appointed by Reagan last spring that they would meet to try to iron out jurisdictional differences.

Six times in the last several months the two chairmen held private meetings to discuss how to regulate investments that don't fit conventional definitions of either securities or commodity futures contracts.

The Shad-Johnson sessions apparently reached a columination about two weeks ago, when the two told other members of their commissions that they had worked out a mutually acceptable plan. With no notice to either the public at large or Congress, the commodity and securities commissions held unusual closed meetings at which a plan drafted by Shad and Johnson was ratified by the two Republican-controlled panels.

The closed commission meetings skirted the federal "sunshine" law that requires government agencies to announce the time, date and subject of meetings in advance so citizens can participate in the proceedings.

It was not even known that the two commissions had formally discussed the controversial issues, let alone made a decision, until last Monday when Shad and Johnson held a joint press conference and disclosed their plan.

While the public was kept in the dark, securities and commodity industry officials were told in advance how the two agencies planned to regulate them.

Johnson invited the heads of the nation's commodity exchange to CFTC headquarters and briefed them on Dec. 4 about the regulatory plan, three days before the agreement was announced to the public, CFTC officials confirm.

Shad held no such formal meeting with the securities industry but discussed the plan with interested Wall Street executives during the months of private negotiations with Johnson, an SEC spokesman said.

It was not until after the public announcement that SEC officials explained the plan to the staff of the House Energy and Commerce Committee, which has jurisdiction over the agency.

That committee's chairman, Rep. John Dingell (D-Mich.), is one of two House panel chiefs who have already expressed doubts about the regulatory plan and are considering holding hearings on it.

Also skeptical is Rep. Benjamin Rosenthal (D-N.Y.), whose House Commerce, Consumer and Monetary Affairs subcommittee last week issued a highly critical report on the CFTC's handling of the silver market crisis last year.

The subcommittee report, written by staff attorney Barbara Timmer, raps CFTC commissioners for holding private meetings with industry representatives. It also criticizes the agency for approving new forms of investment when it lacks the staff and money to oversee the new markets.

Aides said Rosenthal and Dingell have questioned the propriety of the SEC and CFTC making major policy decisions behind closed doors.

Attorneys for the two agencies say the meetings were exempt from the federal "sunshine law" because they were held to discuss pending litigation and upcoming legislation, for which closed sessions are permitted.

Shad, however, has said the two agencies do not need new legislation to implement his agreement with Johnson. If that is true, use of the legislation exemption could be in doubt.

Sources at the agencies also say much of the discussion during the closed meetings had little to do with a lawsuit challenging the SEC's options regulations.

Dingell has complained previously that some of the new investments the SEC and CFTC plan to authorize seem to be little more than gambling on unpredictible fluctuations of the stock market.

At issue in the secret Shad-Johnson talks were new kinds of options and futures contracts that will allow investors to put up a relatively small amount of money and potentially make many times the amount they invest.