I DARE you to suggest that The Undertaker and Kane weren't really ever a tag team. In fact, I dare you to suggest that The Undertaker and Kane weren't really a successful tag team because on that front, The Brothers Of Destruction accomplished more collectively than most fans give them credit for.
The reason why they probably don't get said credit is because of their really unique backstory.
Kane debuted in the World Wrestling Federation nearly 25 (!) years ago as the very-much customized evil doppleganger of The Undertaker. Debuting at the conclusion of the Badd Blood pay-per-view in 1997, The Big Red Machine began a lengthy feud with his "brother" The Dead Man, culminating in big matches at WrestleMania XV and WrestleMania XX (among other moments over the decades.)
Their relationship has been complicated, to say the least (I mean.... just check out this childrens book that attempted to explain their history together). But when The Undertaker and Kane weren't attempting to end each other, they came together to form a badass and underappreciated tag team.
The first tease of a Kane-Undertaker partnership came in early 1998, with a Russo-esque swerve at the end where Kane abandoned his brother during a feud with D-Generation X (which were, ironically, the most recent opponents when the two teamed for in 2018). The pairing never happened, but it definitely planted the seeds.
Later that year, both Undertaker and Kane were both heels, pitted by Mr. McMahon in something of a three-way feud with Stone Cold Steve Austin. And while they did finally team up temporarily to battle Austin, the brothers were still at odds with the WWF Championship up for grabs. It began a pattern where, in the years to come, these two would frequently battle each other in the most gruesome of grudge matches -- and then team up as though nothing had happened.
Over the years, The Brothers of Destruction have battled a who's who of combinations, including The Dudley Boyz, Stone Cold Steve Austin & Triple H, Edge & Christian, Mr. Kennedy & MVP, D-Generation X, Heidenreich & Snitsky, The New Age Outlaws, DDP & Kanyon, KroniK (during their extremely brief 2001 WWE run), Matt Hardy & MVP and The Shield. They also captured the WWF Tag Team Championship twice and at SummerSlam 2001, unified the titles with the WCW equivalent.
Frenemies may be the best term to describe The Brothers of Destruction and their complicated history. But successful frenemies nonetheless.