A federal grand jury in Manhattan yesterday indicted two tax lawyers in connection with an alleged tax-fraud scheme that generated fictitious trading losses of more than $890 million and involved companies in which a convicted Washington tax-shelter operator was a principal.

The 16-count indictment said the scheme enabled unidentified investors to deduct more than $200 million on their tax returns as a result of prearranged transactions in government securities and trading in precious metals.

One of the attorneys is Michael P. Oshatz, 50, of Scarsdale, N.Y., chairman of the advisory board of New York University's Federal Tax Institute. U.S. Attorney Rudolph W. Guliani said that Oshatz promoted various limited partnerships as a means of providing significant tax benefits for investors. The other attorney, Leonard A. Messinger, 47, of Hillsdale, N.J., helped to form some of the partnerships, Guliani said. Both practice in Manhattan and face arraignment on Jan. 7.

The Washingtonian, Edward A. Markowitz, pleaded guilty in April 1985 to tax-fraud charges arising from the sale of more than $445 million in phony trading losses.

The companies named in the indictment were TMG Associates, The Treasury Group, Cambridge Financial Group, The Monetary Group, Ltd., TMG II, ETKO Associates, Komodo Co., B & O Enterprises, Trend Capital Associates and EMESS Associates. Markowitz, the indictment said, was a partner in TMGA, TMG II, Treasury, Cambridge and Trend, and was also president of The Monetary Group.

At the time of Markowitz's indictment 2 1/2 years ago, his criminal tax-fraud case was the largest ever prosecuted.

But while awaiting sentencing, which is set for early next year, he cooperated with and testified for the government in a far larger tax-fraud scheme -- one that generated more than $1.3 billion in bogus trading losses for cash-rich celebrities and other influential investors.

The principal defendant in that case, Charles Agee Atkins, was a former partner of Markowitz in Washington. Last Dec. 10, a jury in Manhattan convicted Atkins of conspiracy and 27 additional charges. Atkins, too, awaits sentencing early in 1988.

Assistant U.S. Attorney Robert J. Cleary said that one count of yesterday's indictment charges Oshatz and Messinger with conspiring to defraud the United States in connection with the operation of the partnerships and corporations.

Both lawyers are accused of causing the distribution of sales literature which falsely told prospective investors that the partnerships would enter into high-risk transactions intended to generate a profit.

Actually, the grand jury charged, Oshatz and Messinger entered into fake transactions that had as their sole purpose the generation of trading losses and interest expenses that could be used as tax deductions.

In order to get these deductions, the various companies, according to the indictment, paid fees based on a fixed percentage of the required losses.

A portion of the fictitious losses was allegedly used to offset fictitious gains that the scheme also created. These fraudulent transactions, according to the indictment, generated more than $50 million in fake tax deductions to be passed on to investors.

In addition, the indictment said, the two attorneys sold more than $150 million in bogus trading losses and interest expenses to other entities and individuals for use as tax deductions.