There is usually a great deal of stretching and distortion to the abdomen after having twins, and in most cases a tummytuck would help greatly to reestablish a firm, well supported abdominal muscular shape. Waiting 6 months afterr your last pregnancy or after nursing would be the advice many OBGYN's offer to their patients, but always feel free to coordinate this with advice from your chosen Plastic Surgeon.
Good luck to you.
Frank Rieger M.D. Tampa Plastic Surgeon
Web reference: http://w.w.w riegercosmeticsurgery.com