completing to ?

She swam as fast as she could, completing two laps to her parents' one.

= She swam as fast as she could, as she completed two laps while her parents completed one lap.

この as she completed っていうところが、主文の主語 she を繰り返してるので、それを省略したくて分詞構文にしているので、completing となっています。

two laps to one のところも、while her parents completed っていうのがくどいので、省略しています。