The subject was one of the larger recent merger proposals, announced just a few days before. Federal Trade Commission Chairman James C. Miller III had just finished telling a group of reporters that it had not been decided whether his agency or the Justice Department would review the deal for possible antitrust violations.

Then he leaned forward, grinned, and said in a conspiratorial whisper, "We'll get it."

When two agencies are empowered to enforce antitrust laws, it's not unusual for them to exercise territorial rights on a particularly juicy case. But insiders at the agencies say that although it sometimes looks as if Justice and the FTC are battling for control of a particular case, there usually is little dispute over how the work load is divvied up.

The process for sorting out cases dates back more than 30 years to an exchange of letters between Justice and FTC officials that set out the ground rules for handling antitrust work.

At that time, the agencies agreed to clear with each other any antitrust work they want to do. In some cases, this is fairly straightforward: Justice, under law, handles criminal antitrust cases, such as price-fixing. Other non-merger situations are handled on a first-come, first-served basis. "It's sort of a finder's situation," an FTC official said. "If you find it, you can work on it."

But the clearance process can get more involved when both agencies might be interested in a case, with no legal restrictions to provide guidance.

"There is obviously tension between the two, in the sense that, at the staff level, people will worry that they're trying to encroach on each other's turf," the FTC official said.

"There are a significant number of relatively gray areas in which we have to spend a little time evaluating each other's claim," said a staff member in the Justice Department's Antitrust Division.

There's no "finder's" situation in a merger. Under the Hart-Scott-Rodino Act, merging companies must file requests for antitrust clearance with both Justice and the FTC. So staff members from the two agencies get together to decide which would be best qualified to handle the case.

"At the first cut, it's divided on an experience basis," an FTC staff member explained. "We seem to have more experience in oil, they seem to have more experience in steel." Thus, the FTC has handled investigations of the possible antitrust ramifications of the recent large oil company mergers, while Justice has taken on the steel combinations.

Justice also gets first claim to mergers in such industries as airlines and banking because the law gives the department more power in those areas.

If the lines of authority still aren't clear, the agencies' staffs each prepare a list of reasons why their agency is better qualified to handle a particular case.

Sometimes, according to one participant, the two staffs split up the work in the simplest way possible. "When there is no better claim, it goes on the basis of you take this one, we'll take the next one," he said. Only occasionally do the staffs reach a deadlock, forcing an appeal to J. Paul McGrath, assistant attorney general in charge of the Antitrust Division, and Timothy J. Muris, director of the FTC's Bureau of Competition.

The time limits that the Hart-Scott-Rodino Act place on the evaluation of mergers make the sorting-out process a fairly rapid one, staff members say, with most cases assigned to one of the agencies within a matter of days.

But because Justice and the FTC leave it to the companies involved to publicize which agency is handling the investigation, its identity sometimes is not known for weeks, reinforcing the impression that there are sometimes long-running "turf wars" over antitrust cases -- which insiders insist is not so.

"It may seem a mysterious process, but it seems to work pretty well," an FTC official said. SECOND-TERM EXODUS? . . .

Antitrust Division chief McGrath isn't the only lawyer in the division who is heading for private practice as the Reagan administration swings into its second term. McGrath's deputy, Helmut F. Furth, also is leaving, to join the Richmond law firm of Baity & Joseph.

There is no sign of a similar exodus at the FTC, but sources say staff members there are delaying decisions on their own futures while they wait to see if Chairman Miller takes a job elsewhere in the administration.