Joint Assault targets only one creature. If that creature is an illegal target when Joint Assault tries to resolve, it won’t resolve and none of its effects will happen. No creature will get +2/+2.

Joint Assault checks whether the target creature is paired only when it resolves. If it becomes unpaired later in the turn, neither creature will lose the bonus. Similarly, if it becomes paired later in the turn, the creature it’s paired with won’t gain the bonus.