FROM ARRANGING flood aid in October, Western-oriented South Korea and Communist North Korea have moved on to open trade talks and to agree to meet early next year to discuss the reunion of families separated by the Korean War of the early 1950s. It is a remarkable surge of public cooperation for two formerly hostile and still deeply distrustful countries whose previous efforts to find common ground had gotten nowhere.

Will this current effort be more successful? It is obvious that South Korea at least has some special reasons for trying to make it so, beyond the familiar reasons of reducing the risks and costs of military confrontation and pursuing the ultimate goal of Korean reunification. Seoul wants to make sure that the 1988 Summer Olympics, which it's putting on, will go smoothly. The Chun government, which came to power by coup, faces another round of dissident challenge and can make good political use at home of its talks with North Korea. As for the North Koreans' motives, we'll forgo guessing. It's enough for now that they are beginning to talk.

But it's only enough for now. Something of substance has to come out of these talks to make the procedure worth pursuing. To put on smiles and to send a few trains of goods back and forth is all well and good, but the real test will come on the "humanitarian" issues that are due to be addressed in two months.

To the Western ear, reunion of families sounds like a lesser issue. To Koreans, it's the heart of the matter. Millions of Korean families were left separated when North Korea invaded South Korea in 1950. Since the war, the border has been sealed: North Korea has entirely prevented exchanges of family information, mail, phone calls, visits and inquiries. The human suffg produced by this North Korean policy is incalculable.

South Korea, a political society that permits its citizens to agitate to resume contacts, attempted to open the family issue in the 1970s. North Korea, which can and does impose the discipline to deny such agitation, demanded an exorbitant political price from Seoul, and the talks failed.

For North Korea, enforcing family division has been more than a diplomatic strategy. It has been the essential method by which a brutal government set out, in the Soviet style, to isolate its citizens and to force its will upon them. So it is no easy thing for North Korea to contemplate visits between separated family members and permanent resettlement from one zone of Korea to the other -- items now on the agenda. It is no easy thing, but it is an essential thing.