I do agree with most of your post. However; before he eliminated one he sexually assaulted her first and then went on to his second victim.
Before he did either, he would make sure each victim was bound, imo. We have read cases of more than one victim, some who lived to tell their story, and many times the person would make one of the victims bind the other one up and then the perp would bind up the second victim then raped them both.
Imo, these girls were both sexually assaulted before being murdered. He wanted both and that is why he took both. IMO.
I think in like 95% of these kind of cases both victims were murdered within an hour and a half time span or less and he was gone from the area before the first 911 call came in reporting them missing.