House-Senate budget conferees wound up a second week of inconclusive negotiations on deficit reductions yesterday, planning to resort to closed sessions next week in an attempt to break a stalemate over the key issues of defense and Social Security.

Senate negotiators rejected a House counterproposal only a day after House conferees rejected a Senate proposal, and each side accused the other of offering only token concessions.

But the suggestion of an "informal" Monday meeting by Senate Budget Committee Chairman Pete V. Domenici (R-N.M.) appeared to signal that, after hours of political posturing before television cameras, the conferees were finally ready to get down to deal-making.

Domenici acknowledged that the talks "appear to be at impasse" but emphasized that an agreement could be reached in a matter of days if basic political hurdles can be cleared.

These include House Democrats' reluctance to tamper with Social Security benefits and Senate Republicans' pledge to the Reagan administration to allow defense spending to grow enough to cover inflation. With the Senate demanding a Social Security freeze and the House insisting on freezing defense spending, neither side has thus far indicated significant movement toward a middle ground.

The importance of these two issues was underscored by the absence of a breakthrough despite progress on other issues. Each side moved more than $1 billion toward the other's position on a variety of domestic programs, but foundered on defense and Social Security.

Budgets approved separately by the two chambers contain $56 billion in spending cuts for next year, adding up to deficit reductions of $250 billion to $300 billion over three years. However, they cut spending in different ways, and the negotiations are aimed at resolving these program-by-program differences.