[Andy]

I can't tell from the OP if the first tag was applied before the obstructed runner reached third base or after the obstructed runner overran third base, or both, so I will have a two-part response


If the first tag was applied prior to the runner reaching third base, umpire should immediately call DEAD BALL! and place any runners at the bases they would have obtained without the obstruction. Since the ball is dead, the second tag (coming back to third) didn't happen.

If the tag was not applied prior to the obstructed runner reaching third base, then the runner runs past third base, s/he has passed the base they are being protected to (third base, if I read your scenario correctly) and are "on their own" and no longer protected due to the obstruction. If the runner is tagged returning to third base, the out stands.