The best - and perhaps most improbable - news of the week is that the Joint Chiefs of Staff may be in for a long-overdue reorganization. It's been talked about before, but nothing has ever come of it.

This time, however, the President himself is reported to have initiated a major study now being quietly undertaken by the civilian Defense Department, which could lead to important changes in the Pentagon's military command structure.

Possibly the most significant change of all would be the removal of one of the two hats that members of the Joint Chiefs wear. In 1947, when Congress unified the armed forces, it provided that the newly created Joint Chiefs of Staff should act in a dual and, as experience has shown, sometimes conflicting role.

Specifically, the JCS is composed of the chief of staff of each branch of the armed services, plus a chairman chosen from one of those branches. Hence, under one hat, the members are called upon to defend and advance the special interests of their particular branch, while, under the JCS hat, they are expected to be above such parochial concerns when they act as the supposedly objective principal military advisers to the President and Secretary of Defense.

It is hardly surprising, then, that the meetings of the JCS are not only planning sessions, but bargaining ones as well. The members have learned the usefulness of composing their differences in private, so that on the big spending items they are generally able to present a united front that the Pentagon civilians, the White House and Congress are not eager to challenge.

It is one of the main reasons for the defense budget's soaring from $70 billion to $120 billion in just the last few years. As one Pentagon official syas, "The Chiefs are a chummy group who work well together, backing each other's projects in a logrolling fashion." In short, "You back my B-1 bomber and I'll back your Trident submarine."

Former Marine Commandant Gen. David Shoup says the officers who rise to the top have usually proved their effectiveness as leaders and planners, "but most of all they have demonstrated their loyalty as proponents of their own service's doctrines."

If President Carter is to make good on his pledge to cut military spending, he and Defense Secretary Harold Bown will have to emulate Robert McNamara who, as Pentagon boss in the 1960s, aimed at centralizing major decisions. According to the respected Center for Defense Information, McNamara tried "to get away from self-set service programs out of balance both among themselves and with available resources."

McNamara's reforms survived in appearance, the Center adds, "but their substance eroded away as his successors turned back to the services the kind of self-determination they had formerly enjoyed."

In consequence, it concludes, "each of the services has developed its own foreign-policy assumptions and builds its programs on internal military premises, rather than on externally set objectives."

In 1958, Congress beefed up the powers of the Defense Secretary, and the impression is gaining ground that Harold Brown intends to make greater use of this added authority. Nevertheless, he will have his hands full, for it is difficult at best to maintain real civilian control at the Pentagon, a major reason being tht Secretaries and Assistant Secretaries come and go before they get the hang of their jobs, while the "brass" just keeps rolling along.

Since the armed services were integrated 30 years ago, there have been 14 Defense Secretaries, the average stay being about two years. In the same period there have been 15 Secretaries of the Navy, 13 of the Army and 12 of the Air Force.

The JCS approval of the Bay of Pigs invasion disillusioned President John F. Kennedy, who later was to say, "The Joint Chiefs advise you the way a man advises another one about whether he should marry a girl. He doesn't have to live with her."

Nevertheless most of the Chiefs I have known have been disciplined and patriotic men, who compare well with the officers of any other nation. It is only natural that they should be fiercely, even narrowly, loyal to their own branch of the service; their problem is how to rise above this when they occupy the JCS "tank." It's not easy.

If Congress removes one of the Chiefs' hats, it might give some attention to a conflict-of-interest problem of its own. There are now about 80 members of Congress holding military officer commissions while voting on defense budgets and military pay and pensions. About 17 members are "double dippers," who receive military retirement pay. It's time, warns the National Taxpayers Union, "we woke up to the perils of a militarized legislature."