Their taste in music and dance is so similar they call themselves Clon, pronounced Clone, but their pre-show warm-up couldn't be more different. While Kang talks to the girls, Ku flexes his biceps by lifting a chair up and down with one arm, showing his tattoo. "Dancing is our life and our weapon," he says, "Movement is everything."
The two 30-year-olds - grandaddies of pop in Korea - have been performing together since their teens. They started as dancers in videos for other singers, including the reggae hit "Excuses" in 1993. Then they won a contest that led to a contract - and they had to start singing.
"When we were kids, we could only hear American rock - at night clubs and army bases. So we copied Michael Jackson, early MC Hammer and Billy Brown," says Ku.
They memorised the songs and the movements of dancers. "We even went to discos and put a Walkman near the speakers to record the mixes, then used the tape in developing our dancing - it is still the rhythm of our music that sells it."
Their first two albums sold more than two million copies - the second has pictures of the London Underground on the cover - and their third, Funky Together, released in the spring, has sold more than half a million. One hit was number one in Taiwan for six weeks, the first song from a Korean band to make it to the top spot.
They are not always popular with the Korean authorities, though. Ku was banned from television for several months for wearing his shirt open to the waist. Now he has to cover up his dragon tattoo. And Kang is forced to forgoe his preferred braids or pig tails for a pony tail.
"On Korean TV, foreign groups like The Scorpions can get away with anything, but older viewers phone in and complain if we flout any rule," says Kang. On tour it is a different story. They do about 25 concerts in a year and all restrictions on dress, hair and lyrics go out the window.
They see their career as work, perhaps unlike Western stars. "We work as if we are salary men," says Ku. "We work hard because of our success. If you could once afford a Mercedes Benz, why would you want to lose it so that you had to take a bus everywhere?" With that he took off his oversized trainers, dropped to the floor and asked one of the girl dancers to sit on his back. Then he started doing press-ups.