The human heart would be under extreme stress in prolonged high gravity environments, at two times gravity your blood weighs twice as much and the heart didn't evolve for those kinds of stresses. A colony could potentially selectively breed and undergo years of natural selection in the hopes that one day humans could be born with stronger hearts, and thicker bones, but those colonists would be born out of decades of suffering, and lower life expediencies. OR you could genetically engineer for those specific traits to allow living on a high gravity world in comfort in far less time.