IN their meetings today and tomorrow in the Netherlands, the leaders of the 12-member governments of the European Community (EC) will pass another important mile post on the route to European unity. The sessions are unlikely to produce the dramatic leaps toward Euro-federalism that some leaders - notably Germany's Helmut Kohl - have dreamed of. But the unification process should appear even more irreversible than it did before the Maastricht summit.The heads of government, in what promises to be a hectic two days, will be trying to work out compromise treaties on both monetary union and political union. On monetary union, the leaders are expected to establish a framework whereby, before 2000, most of the member governments will utilize a single currency regulated by a European central bank. The agreement will be hedged about with numerous conditions and opt-out rights demanded especially by Britain. Still, it seems to be on track. The talks on political union promise to be more contentious. They will cover a wide range of issues, but the key ones are procedural: the role of the European Parliament in establishing EC policy, and the scope of EC "competency" - that is, defining the foreign-policy, defense, economic, social, environmental, and immigration issues on which the EC will be permitted to make binding policy for the member governments on a simple majority vote. Britain, under John Major as under Margaret Thatcher, is the EC nation most loath to surrender any sovereignty and national prerogatives to European institutions. In negotiations leading up to Maastricht, London has made it clear that it will make only incremental concessions on narrow points. But Britain knows that it has much to gain by increased European cooperation and has even more to lose if it is seen as responsible for a collapse in the long-term unification process. Mr. Major, while cosseting the Thatcherites of his party, will be well advised to manifest sufficient flexibility to keep the unification process inching forward. With Eastern Europe in turmoil and Yugoslavia in flames, a strong, cohesive Europe is more needed than ever.