As I said I believe it is likely there is no direct connection. You seemed to have ruled out an indirect connection as well. What good would such a connection be? Probably very little and LE probably wouldn't learn about such until they caught their killer. But what if the Delphi killer had an unusual, and vocal, fascination with the Iowa killings? This, along with any of the signs a killer of Abigail and Liberty might exhibit (e.g., change of appearance, sudden abuse of drugs/alcohol), might further lead to the killer.
As far as a direct connection there is too much distance and too many differences. Also, as you pointed out double murders like this are rare. I believe it was a retired FBI agent stated there were only a total of 15 cases in the US in the last 35-50 years. (I remember '15 cases', but I don't remember the time frame.) That said, what if it is the same killer and they have evolved? Lyric and Elizabeth were ages 10 and 8. That is a big difference between controlling children 8 or 10 and children 14 years old. In both cases the killer was bold. (I like the word 'bold' as opposed to 'confident'. 'Confident' just seems to have a more positive connotation for me and I see nothing positive about these killings.) In both cases this was a very bold killer who either planned well in advance or on the fly and could adapt his plan as the situation evolved. Did he evolve?
I think LE in the Evansdale murders know less than the Delphi investigators due to the length of time the bodies and crime scene was exposed to the elements before being examined. The fact that the lead investigator in the Delphi murders tends to avoid stating they believe there is no connection would lead me to believe there is enough doubt in their minds. And they know more about both cases than the public.
I believe that if this guy has killed before - or since - it was likely a single victim. And that victim may only be classified by LE as missing. That is, no body has been found and therefore no signatures.
And the signatures leads to another discussion. If the Delphi killer purposely left signatures, what satisfaction would he achieve if the bodies were not found? The killer in Evansdale appears to gone to great lengths to ensure the bodies would be hard to find, if not impossible. Such an effort might cause such signatures to be eliminated by weather and animals. A wasted effort in the mind of the killer if he left a signature or signatures. That seems to make the argument that the two crimes are not directly related. Unless, again, a killer evolved.
Right off the only sets of double murders that come to my mind are the ones that were committed not far from me. The Colonial Parkway Murders. And LE is not in complete agreement that all 4 sets are related. The one big argument for a direct relationship that they have is the question, "What are the chances of this many double murders in the same general area within this time frame?" Leaving out the 'general area', that question seems to be one that comes up with Evansdale and Delphi.
