The day after the switch, both couples found themselves in the Sekiharaverse. There, they met up with a very predatory Nico-nii, who preyed on a shrinking, innocent Maki-chan. The couple from the normal world were shocked at the displays of sexuality; those from the OOshimaverse were shocked by who was on top.
When they asked about going home, the Sekihara NicoMaki told them only one ancient, sacred ritual would work.
That's how both Nicos took turns being ravaged by three Makis, though one of them stammered and blushed the whole time. And then both Makis were swarmed by three Nicos. At that point, the three couples formed a Girl Ball where it would be hard to determine who was doing what to whom.
Still, say what you will about Sekihara Umina's NicoMaki, the ritual worked, and both NicoMakis arrived in their own universes safe and sound.
The pure NicoMaki realised they couldn't go back to how they'd been before. They announced their love at the next clubhouse meeting, with tears all around. Somehow, Maki convinced her parents that Nico, two years older according to her student ID, was coming over to help Maki study for her CATs (CBT and OCSE). Technically, the blushing girls were studying anatomy.
The Ooshima NicoMaki decided that both switching off who was on top and denying themselves gratification and putting their relationship on a "pure" basis occasionally would be the antidote to lesbian bed death.
The Sekihara NicoMaki often waxed nostalgic about their salad days having 3 NicoMaki orgies.
And in all three universes, Umi fainted again.