Two runners on one base

haciandia,
Let me try to answer this for you though you really haven't given enough info. If the runner from 2nd was forced to third by a runner being on 1st and the batter hitting a single then the runner already on third, R1, will be out if tagged at third. However the runner who was on second, being forced, will not be out if tagged at third.

In the situation that I think you mean where the runner from second is not forced to go to third, the runner who was already on third, R1, if tagged on the bag is not out but in fact has a right to the bag. If the runner from second is tagged on third while he and R1 is occupying third then, R2, would be out. The way you describe it sounds like R2 was never tagged while both players occupied third and he successfully made it back to second without being tagged. In this case it sounds like no outs are recorded. Remember though, R2 cannot pass R1 or he will be called out. NFHS Rule: 8-2 Art.7 Note

On defense

1 - Tag the first runner
2 - Tag the second runner right away
3 - Tag the first runner again if he leaves the bag

Can lead to double play if first runner thinks he's called out and leaves the bag.