Heatwave had to basically climb Chase in order to make it so the ladder could reach up to he roof of City Hall.
Those two keeping their blance actually was more difficult than it appeared because nither one did something like that and thought they would ruin it at any possible second.
After it did happen, both of them decided 'oh yeah, it could be useful again, we need to perfect it'. From there, both of them practiced carrying each other somewhat like what they did in the episode, but faced many challenges.
One fatal day as they practiced, Heatwave's foot slipped of Chase's shoulder, which led to quite a commotion that was beyond concerning. The fire bot nealty broke his pede from the fall while Chase hit his helm a bit harshly on the floor to nealry be concussed.
Chief banned them from doing it again.