Couples definitely have an advantage when they work together.
A study published in the Journal of the American Medical Association and conducted University College of London found that couples who undertake a fitness program together were three times more likely to succeed at their goals than those going it alone. The Study, entitled: The influence of partner’s behavior on health behavior change: the English Longitudinal Study of Ageing, looked at 3722 married and cohabitating couples. It focused on three areas of lifestyle or behavior: smoking, exercise and obesity. In all areas, the study found that when one partner in a couple changed to a healthier behavior, the other partner was more likely to make positive health behavior changes than if their partner remained unhealthy.