DaleB does bring up a very valid point.  Except we would speaking about 2 large starships capable of sustaining a crew each of hundreds of individuals.  Each vehicle self-sustaining and fully equipped.  Gardens for food, sleeping quarters, machine shops for manufacturing, laboratories for research and development.  Of course, some logistics must be addressed like sanitation and waste management, long-term medical and chronic care, child birth and death.  Remember, these missions will be long-term, perhaps spanning 10 or more years.  And if something goes wrong, there is no instantaneous return to Earth.  The vehicles would have to sling-shot back for an emergency return.  That is the reason for 2 ships rather than 1.