As a Roblox developer, it is currently impossible to remove a player and re-enter the same player in the Multiplayer simulation:
If Roblox is able to address this issue, it would improve my development experience because sometimes it is necessary to test the behavior of the game when one player is online and another player enters and leaves with the first player still online.
Currently, this is only possible in Roblox Player, because Studio starts ALL virtual players at the same time.