That's all very logical, but you're thinking like a sane person
The most obvious reason not to carry them in together is that the person wasn't physically strong enough to do it that way. They were both big girls, around 90 lbs. Carrying both of them together would require quite a bit of strength.
This person would have to have the strength of superman to carry them both at the same time. This is another reason I feel it was one perp and not two. If two people did this, then I could see two people carrying them in, walking together, laying them together.
I would turn my thinking around to the possibility that they walked in and were killed there if it were not for a couple of reasons. The first one is the fact that it was referred to as a dumping ground, yes by the media which you can not always trust, but I feel this person may have been close enough to have heard a lot of the initial talk that was going on the day of discovery.
The second reason is the convenience of the whole thing, along with the timing, things were also convenient for this person. Down to the fact that Seven Bridges was only twenty miles away and not used for family picnic's anymore. Yes it was used for hunting but not until the fall. It would not have been convenient for this person to spend the time to dig a hole, in the dark, to bury them. If a hole was dug in the day light, they would be risk of being seen. How close is the next property from where they were found? Someone could have been planting their field. Spend the time to do that for what reason? They were in an area that wouldn't be used much until hunting season anyway.
I wish we knew which child was further in, I would think it would be Elizabeth who was the smaller one of the two and easier to carry.
