There is no rule against a single checker hitting two blots in one turn. At backgammonrules.net (emphasis mine):
If a player moves to a point occupied by only one of his opponent's checkers, the checker is hit. Hit checkers are forced to re-enter. This slows their owner down. The opponent proceeds - and a player ahead in the race home is more likely to win.
- Keep an eye on points occupied by a single checker, since these checkers are blots and can be hit.
- Hit when you can, but weigh the risk of thereby leaving a blot yourself.
- Opt for a two-stage move to throw out two blots at once or hit one and move on to a safe place.
- Place the hit blots on the bar.