ANYONE WITHOUT a long memory for these things is liable to take for granted what is actually an extraordinary event, the agreement on Monday -- before the president acted on the issue -- between Senate Majority Leader Howard Baker and House Speaker Tip O'Neill to seek, jointly, a gas tax increase to finance a roads-and-jobs program. It's not unusual for politicians to back a highway program. But it is very unusual for the Democratic leader of one house and the Republican leader of the other to announce a common policy. Consider the contrast with the last time the two houses of Congress were in different parties' hands, after the 1930 election. House Democrats were then busy trying to impeach the secretary of the Treasury, while the Republican leadership of the Senate was determined to hang in there with President Hoover.

One of the reasons for this extraordinary spectacle is that both Mr. Baker and Mr. O'Neill were strengthened by the election results. True, the proposal they are backing is to be considered in the lame-duck session that starts next week; but even in the old Congress, the power balance of the new one will be reflected. Those who have been strengthened will drive harder bargains, and those who have been weakened will not necessarily be consulted. The O'Neill-Baker agreement was reached, apparently, without detailed consultation with the House Republican leader, Robert Michel, who was reelected himself by a narrow margin and lost many of his followers, or with the Senate Democratic leader, Robert Byrd, who has not been able to muster his minority in disciplined array on major issues. Instead, the leaders who have shown themselves capable of -- and interested in -- assembling majorities have gone to work, quickly, and have tried to address what they consider to be a pressing issue.

You don't have to accept every jot and tittle of their plan, nor do you have to consider it really a major contribution, to applaud their collaboration. It is more important as evidence that these two men are more interested in constructive action than they are in partisan maneuvering.

Still ahead are the tough issues: Social Security, the budget, taxes. Agreement is not automatic, even with all the good will in the world. Certainly it will not be easy to achieve without active participation of the executive branch. But the agreement reached by Mr. Baker and Mr. O'Neill shows that there is a potential for a bipartisan, two-house government, even if Mr. Reagan should choose, as he chose for much of 1982, to avoid the unpleasant arithmetic and the hard decisions on the big issues. That's an encouraging sign, and one that should not be taken as business as usual.