Yes, it would still be a double homicide. First of all, one of them can't tie the other up...then tie themselves up... and would be a lousy way of committing suicide. In general, I think if you restrict someone's ability to save themselves then it is a homicide...not sure what degree. Let's say you throw someone who is alive into a roaring river and since they can't swim well, the drown... that is murder. Also, you dump off someone without water who is alive into Death Valley and they die of dehydration. that is also murder.
Anyway, this was crazy thought I had because my gut tells me someone got pissed at these girls from some reason and wanted to teach them a lesson one way or the other. So, what if they were found tied up to a tree and they otherwise only froze to death?