While it may not have been implemented in any Mario Kart game outside of Double Dash, the idea of having two racers per kart has not been forgotten by Nintendo -- and it may be seen again in the future.
When asked during a GDC roundtable why Double Dash's most noteworthy innovation has been left out of subsequent entries, Mario Kart 8 producer Hideki Konno explained it wasn't a matter of disliking the idea.
"It wasn't just a simple case of, 'Hey, we're not going to do that again,'" Konno said. "We really think the two racers together in one kart of Double Dash was really unique, and we thought it worked really well."
What it comes down to is needing new ideas with which to make use of the two-racer setup. "We've got that in our pocket, so if we come up with any new, cool ideas that utilize having two players racing together, we will definitely grab that and possibly bring it back out," Konno continued.
There's also the technical side of all this, as Konno mentioned one of the the challenges involved with having multiple racers per kart that isn't apparent on the surface.
"Having two players racing together, it does have a pretty high cost in terms of processing power, so that is an obstacle," Konno said. "That being said, if we could come up with some ways around that processing cost, we may have two players, maybe even three players, racing together. If we could come up with some cool ideas, we'll definitely use them."