In an unusual move, Secretary of Defense Caspar W. Weinberger has established a second scientific panel to study Dense Pack, this one to criticize the controversial land basing program favored by the White House for the planned MX intercontinental missile.

The new group of experts, according to a Pentagon spokesman, has been directed to come up with all the negative aspects of the plan, particularly how the Soviets could overwhelm it. Such information is expected to provide new ammunition to foes of the plan, and could spell its doom before it gets under way.

Just little more than a month ago, Weinberger established a Defense Science Board panel to determine how Dense Pack could be made to work. The administration had been pushed toward Dense Pack by members of Congress dissatisfied with the administration's move to put MX in existing Minuteman silos.

Under the Dense Pack concept, the latest in dozens of basing options, the giant new ICBMs would be placed close together in cement-hardened shelters, with the idea that if the Soviets tried to knock them out with a barrage of nuclear warheads, the first one to hit the ground could destroy one U.S. missile. But the explosive force, debris and radioactive fallout created from that blast would disable other incoming Russian warheads and prevent them from blowing up the remaining American missiles.

The idea of one exploding nuclear warhead destroying others aimed at the same target is called "fratricide" in nuclear weapons parlance. It is, however, a theory and one not accepted by all scientists.

Weinberger, according to administration sources, does not now favor Dense Pack, and the plan's supporters in the Pentagon and White House attempted to halt formation of the second advisory panel. "It will provide ammunition for MX opponents on Capitol Hill," one administration official described as the argument against setting up the new group.

The first Defense Science Board panel, which held an organization meeting Friday in the Pentagon, is headed by physicist Dr. Charles Townes of the University of California.

A group headed by Townes last year that studied various basing proposals for MX took a brief look at the Dense Pack idea, according to informed sources.

On paper, Dense Pack appeared to solve one of the major stumbling blocks for any new MX basing plan. It would not take up a lot of land. As planned, it is said 100 MX missiles could be packed into a 10- to 15-square-mile area.

To avoid running into problems with environmental impact studies, the Air Force is looking at placing Dense Pack missiles on an established military reservation, such as Warren Air Force Base, Wyo.; Nellis Air Force Base, Nev., or White Sands Missile Test Range, N.M.

Proponents of Dense Pack see the first package of 100 missiles and 100 shelters as only a starting point. If the Soviet threat appears great enough, they foresee building a second set of 100 shelters in another state, and providing the 100 MX missiles with a deceptive basing capability by putting some of the them among the 100 shelters.

In a third phase, an antiballistic missile capability could be added, under current planning.

To meet another criticism of Dense Pack, that it violates the prohibitions on building new fixed ICBM launchers contained in SALT II and the 1972 interim agreement on strategic weapons with the Soviet Union, sources said the MX missile will be "encapsulated." That means each MX will have its own launcher, will be able to be moved from one shelter to another and thus qualify as a mobile missile, which SALT II permits. The new hardened shelters would not be considered "launchers" under this approach.

Uncertainty over how the MX missile is to be based may lead Congress to refuse production funds for the missile in the fiscal year 1983 defense authorization bill. The Senate took that course last month and Pentagon officials fear the House may follow suit when it takes up the bill after the July 4 recess.

It was in part to give the House the belief that the administration was settling on an MX basing plan that the White House late last month released the information that the president was leaning toward Dense Pack.

In fact, Dense Pack is still in undergoing theoretical reviews.

At about the time is was being described as the new favorite in the White House, scientists at Los Alamos Scientific Laboratory, N.M., where nuclear warheads are designed, were being asked by the Pentagon to undertake studies on the effects of "fratricide," sources said.