In addition to Jesus, there were several false messiahs appeared in the first century. Each of this examples are from humble origins (shepherds, etc.), sought to set themselves up as kings, and developed a peasant following.Chronology, even fairly recent chronology, was difficult to get right in antiquity. I think it is more likely that Luke was a little mixed up on the date of the rebellion of Theudas than that there were two Theudases rebelling within a decade or so of each other. But anything is possible.
Crowd numbers are very difficult to verify even now, so it is no surprise that Josephus and Luke give different estimates of the size of the crowd that was following the Egyptian.
Past posts in Phil Long's series on the Second Temple Period are noted here and links.