What are the two parallel lines between these notes called on flute?

Chances are, it's a tremolo - and chances are equally good that the composer or arranger wrote something unplayable - at least by the majority of HS kids (depending upon tempo and other factors, like their general flexibility.) If they want a tremolo between D3 and A2 - then there is a limit on how hast you can go between those notes, using the regular fingerings. If every other player reversed it - chair 1 plays D-A, chair 2 plays A-D - sometimes you can get enough of the effect. Some tremolos need harmonic fingerings. Best source is www.wfg.woodwind.org. If you post a photo of this, I can help better.