Place potatoes and kohlrabi in a large saucepan and cover with water. Bring to a boil over medium-high heat and then reduce heat to maintain a rolling boil. Cook until potatoes and kohlrabi fall apart when poked with a fork. Remove the potatoes and kohlrabi from the heat and drain off the water. Mash and add milk, garlic, butter, salt and pepper; stir to combine.