The Team-Up ability is a unique ability introduced in Donkey Kong Country 2, and has been featured in some games since. It generally involves carrying an ally on his or her back. Donkey Kong can be seen carrying Diddy Kong in some artwork for the original Donkey Kong Country, but this was not made a full-fledged ability until the sequel.
Donkey Kong Country 2
With the Team-Up ability in Donkey Kong Country 2, Diddy Kong and Dixie Kong can use it to gain some vertical and horizontal distance. Instructions for the move are incorporated into the first level of said game, Pirate Panic. While teaming up, both ground movement and jumping are hindered to a small extent, but the added weight could be used to defeat some of the tougher enemies such as Kruncha. Alternately, a Kong can be used as a projectile to harm enemies or reach items and areas from a distance. However, if the tossed Kong does not reach anything, he/she will not go back to the player, merely showing a hurt status. Walking over to the Kong will get him/her back up.
Though it didn't matter who was riding on top of who, Dixie will show more trouble picking up Diddy, and when Diddy picks up Dixie, she enjoys the ride and points towards random place to go. These animations are intended to be humorous and do not influence each other's abilities.
Donkey Kong Country 3
In Donkey Kong Country 3, the Team-Up ability returns. This time around, Dixie and her cousin Kiddy use the ability similarly to DKC2, though with some subtle differences. For example, Kiddy is considered a heavy counterpart to Dixie, and when thrown can be used to break certain areas in the ground. To counteract this, Dixie walks considerably slower than usual, and cannot throw Kiddy very far. She can toss Kiddy to the left or right, ricochet him off a wall and ride him much like a Steel Keg (pictured). It's worth noting this would later be developed into the Kong Roll in DKCR. When Kiddy picks up Dixie, he can toss her far and can use her like a projectile.
Once more, the team-up ability does not appear in the game's follow-up Gameboy title, Donkey Kong Land III because of the poor capabilities of the Game Boy.
Donkey Kong Country Returns
Teaming up made another appearance in the series' return to its roots, Donkey Kong Country Returns. Its functions are different from before. In the game, Diddy cannot use the Team-up ability to pick up Donkey Kong due to DK being too large. Thus, Donkey is delegated to carrying is little buddy throughout the adventure. In single player mode, when a DK Barrel is found, Diddy will automatically go to DK's back. This doubles their endurance (and restores any of DK's lost hearts) and gives them new abilities.
During the Team-up, Diddy can use his jetpacks on DK's back to hover in the air with him. Diddy can also fire his Peanut Popgun from his partner's shoulder. He can also use Donkey Kong for the Kong Roll ability, allowing the two to plow through hordes of enemies and go through a straight plain in a level quicker. However, if two hits are taken while Diddy is on, he will be too injured to continue.
Donkey Kong Country: Tropical Freeze
The Team-Up ability returns in Donkey Kong Country: Tropical Freeze, expanded to incorporate the new playable characters. Donkey Kong can have Diddy, Dixie, and Cranky Kong on his shoulders. Diddy is essentially the same as he was in Returns, Dixie can use her Pony Tail Twirl while on DK, and Cranky can use his cane to bounce off of enemies and spikes while on DK.
Official artwork of Kiddy carrying Dixie in Kremwood Forest.