During the 1000 years, people will be born and die, although their life span may be significantly longer than present. Some of those people will embrace Jesus and the gospel of salvation during their life's.But the second one is not a mix of saved and unsaved. The second one is only for the unsaved and one clue is they are called the dead despite their resurrection having taken place. This is because they are still dead spiritually after the physical resurrection. They are physically resurrected in order to be able to die a second time ie: the second death which has no resurrection.
Thus, when the general resurrection for the GWT judgment take place, they will be among those resurrected. And those saved people will receive their eternal life bodies at that time.