When the Pittsburgh Pirates and Montreal Expos meet, one day simply isn't enough to settle their differences. They just keep on slugging each other all night and into this next morning.

And even then, nothing is decided, as was the case tonight in their splendidly split twinlight doubleheader.

Seven days ago, these foes played until almost 2 a.m. in Montreal. On this night, their warfare wasn't over until 12:40 a.m. as the Expos, knocked out of first place in their 5-2 loss in the opener, scored the inning run in the ninth inning of the nightcap for a 7-6 win that pushed them back in front by a half-game.

This looked like the battle in which the Expos would finally expire. The Pirates led, 6-2, in the second game, and Montreal, which now has played six doubleheaders in 10 days, seemed dead on its collective feet.

While the Pirates were turning six sparkling double plays during the evening, the Expos were making six errors, including three in the second game by Dave Cash.

Yet, somehow, the Expos came struggling back like a dazed, out-on-his-feet fighter.

The eighth inning of the nighcap was a condensation of the Expos' entire season as they scored three runs to tie the game with the Bucs' relief master, Kent Tekulve, on the mound -- the same diabolical sidearmer who had saved the first game (his 30th) with three perfect innings.

Did the Expos rely on their quintet of sluggers, all of whom have from 72-90 RBI? Not on your life.

Who but unknown catcher John Tamargo, pinchhitting for little-known Rodney Scott should ground a double over the first base bag off Tekulve to drive in the two runs that tied the game, 6-6.

That swat, which may have kept Montreal's division hopes alive with two more head-to-head showdowns on tap here, was a testimony to Expo Manager Dick Williams, who has never been more brilliant than in the last fortnight.

While Pittsburgh Manager Chuck Tanner had old Willie Stargell still playing defense in the eighth -- poor thinking it proved -- Williams had thought ahead. He had taken his hottest slugger, Larry Parrish, out for pinchrunner Tim Raines.

Had Parrish been on first, he never would have scored on Tamargo's double -- not against Dave Parker's arm. But Raines slid across with the most vital run of the night.

In the ninth, the tension of imminent defeat had been removed and the Expos, who have won 31 to 41 games since Aug. 15, went about their business like workmen.

Ancient Rusty Staub drew his fourth walk of the game, took second on a single by cather Duffy Dyer (only playing because Gary Carter jammed his thumb in a plate collision in the fifth), then headed for home as Ellis Valentine's grounder trickled into right field.

Certainly, Valentine should not have been the Expos' hero. He was 0-for the doubleheader, and one for 17. But he delivered. And Staub should have been the last man on earth trying to score from second on Parker.

But Le Grand Orange lost many a calorie as he dug and scrambled his way around third, wallowed across the plate, then walked ever so slowly back to his bench with his baby-blue uniform as dirty as even Pete Rose could wish.

The Expos' new day was almost impossible to believe. From 6 p.m. until midnight, they were a pumpkin.

Once, the Pirates were a fun team -- fun at bat and even more laughes in the field. Now, they have become old-fashioned fundamentalists afield, veritable party-poopers who turn rally-killing double plays and make one special catch after another.

And it was the Expos' party that they were pooping for the first six hours tonight.

Both games reached the middle innings tied, 2-2. And both times all the Pirate runs from that point onward were extremely tainted. While the Bucs were saving their pitchers, the Expos were burying theirs.

"The change in this team is our infield defense," said Tekulve, who preserved Bert Blyleven's first game win. "It used to be surprising to turn around and see what was going on out there behing you.

"Now, the only surprise is what good plays we're making."

The other renowned Buc teams of the '70s, the Lumber and Lightning Companys, would not have believed the twin-killings turned by Tim Foli and Phil Garner.

Conversely, the Expos' blunders were just as heinous. In the opener, reliever Stan Bahnsen fielded a simple hopper as a Pirate headed home, only to find that the ball had turned into a snake. He dropped it -- purely a victim of pennant-race fingers -- as the go-ahead, and eventually winning run scored.

When Valentine in right field was not playing a bloop single into a triple, catcher Carter was playing tough-guy, let's-bump-heads baseball with Bill Madlock at the plate, and dropping the ball to allow an insurance run.

The second game was one long Expo faux pas. Cash managed to make two errors on one play -- kicking the ball, then throwing it wildly. He topped that by booting a grounder with the bases loaded, opening the doors to a four-run Pirate inning.

Espos starter Rudy May probably doesn't know which mate to kick in the shins first.Larry Parrish dug him a hole by turning a perfect dougle-play, perhaps around-the-horn triple play grounder, into just one out -- a play that also led to Pirate scoring.

But after midnight, the Expos played like Cinderalla with an all night pass. After one of the most brutal stretch-run schedules in baseball history, and with seven more games in the next days facing them, they began whooping and cheering, begging for their chaps named Tamargo and Raines to whip the mighty Bucos.

If ever two teams were both under praised, yet deserving of the highest baseball adulation, it is these two. While the Expos have set a withering pace, Pittsburgh has won 22 of 30 games. No team in the NL can so much as slow down these two budding powers which only found their personalities at midseason.

Each club has 94 wins and they may both end up in the exalted vicinity of 99 victories.

In fact, if these two teams should have to decide matters next Monday in a playoff game in Montreal, it might be a fitting showcase for those Expos and Pirates who have not captured the nation's full attention.

There's only one problem. They'd probably play all night.