When people talk about DNA results in a couple of days, they are talking about very specific scenarios that many not apply here. (I know that many people here know this, but others do not.)
Upthread, someone mentioned the rape of a child. If DNA in the form of semen is found in the vaginal vault of a child, there is no alternate explanation for that DNA. The only conclusion is that it belongs to the rapist. That's a DNA test that could be completed in days. All that's required is a clear DNA sample recovered from the semen and a clear swab from the suspect. Compare A to B. Done.
That's a best-case scenario (speaking from a crime-solving perspective only). In this case, we're dealing with two victims, and it's possible there was no sexual assault. Also -- I apologize for being indelicate -- there are plenty of sexual assaults that don't leave DNA behind. The presence of semen -- and again, I'm sorry if this is overly blunt -- is not conclusive evidence of sexual assault. No seven-year-old is having consensual sex, but a fourteen-year-old might or might not.
But let's postulate, for a moment, that there is no biological evidence of sexual assault. Lots of reasons that could be the case. All of a sudden, what it means to test DNA in this case becomes a lot more amorphous. If trace DNA is present in the form of hair on the girls' clothing, touch DNA on their shoelaces, this is getting way more complex. Imagine that LE now must test dozens or even hundreds of complete or partial DNA samples that could be innocuous or could be from the killer(s). They've got to test against and rule out every member of both girls' families, friends, each other. If you're using DNA testing not to determine whether A matches B, but rather, which of these traces is the outlier, that's not going to take three days.
Of course, all this is my opinion only and I have no way of knowing if this is or isn't the case here, but this is all to explain why I don't think any conclusions can be drawn about DNA testing just yet.