IN THE NAME of stability, the Pentagon is now testing the idea of a two-year instead of an annual budget. Stability is a worthy goal, but a two-year budget is the wrong way to reach it. Congress would lose too much control.

The idea has been around for a long time. The Armed Services committees provided in last year's defense authorization bill that the department submit a two-year budget beginning next January for fiscal 1988. The submission would be an experiment. Congress has not agreed to approve a biennial budget, just to receive one.

*The theory is familiar. The members are now said to spend too much time fine-combing and -tuning the budget and too little time setting goals and strategy. A two-year budget would force and allow them to back off and to think of ends as well as means. At the same time Congress is said to be inconstant in its attitude toward defense, voting large increases one year and small the next, so that rational planning and efficient production become impossible. A two-year budget would provide fewer opportunities to veer.

All true, except that:

When the Pentagon complains about congressional micro-management, what it has most in mind are not micro issues, but major ones. It is not the buzz (which will persist in any budget cycle) over whose constituents will supply coal or milk or uniforms, but the basic disputes over whether and how to proceed with new research and weapons programs -- DIVAD, Bradley fighting vehicle, AMRAAM, Osprey, MX, C-17, SDI. These are not issues from which Congress should withdraw.

Nor is Congress the only source of the erratic pattern of defense funding in recent years. The Pentagon has pressed relentlessly for more, packed and spring-loaded the budget and tried to lock in large increases instead of planning for smaller and steadier ones. Who believes it would behave differently on a two-year cycle?

The Pentagon already does a great deal of long-range planning, and Congress a fair amount of long-range appropriating. One reason it is hard to cut Pentagon spending in any year is that so much of it derives from prior decisions. For fiscal reasons Congress needs to retain flexibility and control over spending. A two-year budget would help to insulate the Pentagon from fiscal considerations. The effect would be to put more of the fiscal burden on domestic programs. That is the wrong way to go.