The Navy has identified two Poseidon missile-launching submarines, the USS Nathan Hale and USS Andrew Jackson, for dismantling next year if President Reagan continues his policy not undercutting the unratified, SALT II agreement limits, congressional sources said yesterday.

That action would be required when the new USS Nevada, a Trident submarine with 24 multiwarhead missiles begins sea trials next May. The Nevada would push the United States above the SALT II sublimit for multiwarhead missiles by 22 missiles. Since Poseidon subs only carry 16 missiles, the United States will have to take two Poseidons out of service to avoid violating the pact.

Reagan is expected to make his decision on what to do about the submarines and SALT II before Dec. 31, when the agreement expires, according to administration officials.

Also affecting Reagan's decision will be a Defense Department report on Soviet treaty violations that he received just before last week's summit. The Pentagon is drafting recommendations on what the U.S. response should be to those violations.

Still smarting from the presummit leak of the violations report to The Washington Post and The New York Times, a Pentagon official yesterday refused to say when those suggestions would be sent to the White House.

Last Friday, Reagan told a news briefing that he had discussed SALT II with Soviet leader Mikhail Gorbachev in Geneva and "made it plain" to the Soviets "that we certainly were not going to bind ourselves to something that was not equally binding on them."

Last spring, a major interagency battle, featuring Pentagon civilians on one side and State Department officials on the other, preceded the June 10 decision by Reagan to dismantle the Poseidon submarine USS Sam Rayburn. At that time, the Pentagon opposed dismantling the Rayburn because of Soviet violations of SALT II.

In the end, however, the decision was made to "go the extra mile," in the president's words, to stay within the SALT II sublimit.

Yesterday, a key administration official said the coming interagency discussions of what to do about the SALT II limits "will test the spirit of Geneva" -- not between Washington and Moscow "but between the Defense Department and the State Department."

At the Geneva summit, Assistant Secretary of State Rozanne L. Ridgway and Assistant Secretary of Defense Richard N. Perle worked well together in negotiating with the Soviets over the joint statement issued by the two leaders, according to informed sources.

The Soviet Union sought to include in the summit statement a one-year extension of the SALT II no-undercut policy, but the United States refused to accept that language.

The Soviets were told that the president would continue his policy of respecting the treaty provisions provided that Moscow exercised comparable restraint and pursued arms-control negotiations.

In a letter to Reagan shortly before the summit, Defense Secretary Caspar W. Weinberger stressed that the president should resist pressure to extend U.S. adherence to SALT II because it would prevent an adequate response to Soviet violations.

Reagan said Friday that "we have found 23 violations of the SALT II agreement and these are things that have to be cleared up between us."

In a related matter, administration sources said yesterday that the Soviet Union has reduced the number of its operational SS20 missiles in Europe to 243, as Soviet leader Mikhail Gorbachev promised in Paris last month.

However, the reduction of 54 of the mobile, intermediate-range nuclear missiles was done in a way that would permit them to be quickly reactivated, one U.S. official said.

The Soviets, this source said, had only partially dismantled facilities for some SS20s, at nine regimental sites. He said it was uncertain what had happened to the missiles.

Since the remaining elements in each regiment are intact, the source said, it would be simple to reconstruct the full unit and return the dismantled missiles.