Two of the government's biggest and certainly most powerful -- unions are talking merger. That is, they are talking about talking about merger.

A linkup between the two AFL-CIO giants, the American Postal Workers Union and National Asociation of Letter Carriers could give them a hammerlock on the nation's biggest, and one of its most vulnerable communications networks.

The APWU represents most of the "inside" employes of the 600,000 members U.S. Postal Service. The Letter Carriers union has most of the "outside" employes of the USPS. Eight of every 10 employes eligible to belong to a union carry an APWU or NALC card.

In addition to the tentative merger feelers, representatives of the two postal unions have long-standing contacts with the Communications Workers of America. A linkup between those two, and the CWA, could give the new union the same clout the Teamsters enjoy on the highways.

But none of the union leaders expects to be sharing an office with any of his fraternal rivals anytime in the near future.

Although allies under the AFL-CIO banner when facing the common enemy -- Postal Service managment -- the two unions are miles apart in many respects. Letter Carriers union officials have alway thought of their union as the shock troops, or marines, of the federal labor movement. They traditionally have felt, in private, that the APWU, which has as many national officers as some locals have members, is top-heavy and spinning in too many directions because it represents different crafts.

The AWPU, which was created by a series of mergers of craft-oriented unions, traditionally has considered itself more democratic and less subject to one-man rule than the Letter Carriers, who cherish a tradition of strong leaders.

The rivalry between the two big unions is nothing new. In fact, first-term Letter Carriers President Vincent Sombrotto and veteran APWU leader Emmet Andrews probably get along as well as any of their counterparts in recent years. Andrews last year survived a strong election challenge by militants who said they wanted a harder line against the Postal Service even if it meant a strike.

True to their militant tradition, the Letter Carriers last year ousted their incumbent president (who called for moderation) in favor of Sombrotto, who ran as the hard-line, get-tough outsider.

The other problem to this or any other union -- or corporate -- merger is what to do with incumbent officers. Brothers aside, being the national officer of a postal union is better than returning to a mail route or letter-sorting machine in a post office. Merger would mean somebody (lots of somebodies) would have to go, and both unions have young, ambitious officers here and in the field eager to get or hang onto top jobs.

Postal managers have mixed feelings about the prospect of dealing with one big union. It would mean a more powerful adversary, but some think a single major union -- in addition to the influential Mail Handlers and Rural Letter Carriers -- could make negotiations move quicker. Whatever the benefits, merger isn't in the cards anytime soon, no matter how many merger committees are formed and how many meetings are held.

"If you think the Camp David talks (between Egypt, Israel and the U.S.) were tough, imagine arranging a marriage between us," one of the union leaders noted.