Actually, the two were unfortunately included in the game, and Garrick tried to be heroic and fight his way out of it. Obviously, this doesn't work. Fortunately, though, they don't get summoned in Olgar's story, which means that in Olgar's alternate timeline, they probably weren't strong enough to qualify, so they make do with four.
Although this makes sense, this contradicts what the Almighty says, which is that it selected and summoned the five couples to compete in the arena.
Two more possibilities here. One: They are intended to weed out any contestants who might oppose the tournament, or refuse to create the human race should they win. Two: Olgar and Vahti weren't initially supposed to compete, but Ayuta gave them a second chance, replacing Vahti with a Theia clone in order to attempt to force a couple capable of creating a human race strong enough to defeat the Qualia to win.
Considering how rebellious the last set of contestants were, perhaps they brought an extra couple just in case.
Did you feel sorry for killing all of those people? Fear not! At the true end of the game, the Almighty gives everyone the option of returning home at the time they choose. Everyone chooses the point before they were summoned. This means that everyone who they ever killed in the tournament is now back to life. Except for the Qualia, because it was killed permanently at a point where all time converged.
Related to that last point: everybody chooses to return at the exact same time they left, including Zhamo. Eluca is surprised at this seeing as she was about to execute him at that point but he brushes it off saying that they can work together to bring down the Divine King now. Pretty noble except for one small detail: earlier in the game, they explicitly say that Eluca was about to execute Zhamo...after she had already killed his parents. And he wants to go back to a point literally moments after they were killed? WHAT THE HELL?