When two runners are on the same base, neither one is automatically out. However, depending on the circumstances, only one of them has a legal right to the base. The other runner may be tagged put, even if he's contacting the base.
Which one can be tagged out? If the lead runner was forced to advance, then he's the one you can tag. If the lead runner was not forced, then the trailing runner can be tagged.
On the play in question, I assume that there was a batted ball and the batter-runner is now standing on first. If that's the case, then the lead runner at second is forced and he's the one that can be tagged out.
Otherwise, if there wasn't a batter-runner on this play- like a botched double steal- then nobody is forced to advance and the trailing runner is the one you need to tag.