Two Runners Occupying a Base
Two runners are not allowed to occupy the same base.
If two runners are touching the same base, the lead runner is entitled to the base. Most coaches will teach their defensive players to tag both runners when they are occupying the same base. When this happens the lead runner is safe and the other runner is called out.
With all things being equal (meaning I wouldn't have a player run around one runner without tagging him to tag the other), I tell my players to tag the following runner first. The reason for this is: If they tag the lead runner first, and the other runner starts to go back to the other base before he has been tagged, no out has been made. We now have a rundown and no out. As you know, especially in youth baseball, anything can happen at this point. If the defensive player touches the following runner first, he is out as long as the lead runner is still on the base. If the lead runner takes off then at least we will have a rundown to get the lead runner rather than a rundown where the players have to keep an eye on the lead runner.