1. A computer-implemented method, the method comprising: sending by one or more computers independent queries to a travel planning system for each of a plurality of passenger groups, the independent queries being for at least two different passenger groups sharing a co-dependent segment of travel; receiving lists of individual solutions appropriate for the passenger groups in response to the independent queries; combining by one or more of the computers, the lists, to provide joint solutions according to joint travel requirements/preferences, with joint travel requirements/preferences corresponding to requirements/preferences of at least the two different passenger groups sharing a co-dependent segment of different travel routes for the two passenger groups, with a co-dependent segment having the same origin, destination and travel time of a portion of the different travel routes for the two passenger groups; determining by one or more of the computers if there are a sufficient number of joint solutions that are travel itineraries that satisfy the joint travel requirements/preferences; and, if there are a sufficient number of joint solutions, return the joint solutions.