Just did this in the sim. Started at FL350, .82 / 340KT descent, got engines started at FL200. Engines restarted before the APU was available. I disagree with the checklist in attempting APU starts immediately as you are wasting battery power, and it will not suction feed start above FL250 anyway. The inflight relight was exceedingly slow and could easily be unintentionally interrupted by cycling the fuel lever.