It's a fairly complicated and very fact-specific issue. Different states and the federal courts have different ways of handling it, and I don't know Virginia's specific rules, but yes, at least in some states, it is possible to consolidate multiple murders that were not part of the same act.
But even if the charges were tried separately (which, I bet, would be the likely outcome, if JLM were charged with both offenses) then evidence of the prior attack might still come in, depending on what evidence they have. If something about Morgan's murder could show the identity of Hannah's abductor/killer, for instance, then that sort of evidence could be admissible. But the judge could also decide it is just too prejudicial.
Tl;dr: it depends on the nature of the connections between the crimes, and whether there is some relevant similarity between them that would make evidence of the first murder admissible in a trial of the second (or vice versa).
I agree. Most of the time, DAs do not want to combine cases from what I have seen unless it's all the same episode, such as the Loud Music case, where the perp, MD, shot at a van with 4 young men it it and killed one of them. Instead of trying him for each count, against each of the men, the DA took it as an all in one. The case is being retried, as of now, by the way, a portion of it anyways.
It can get too confusing to try to mix both cases in one trial, especially when they are separated in time. Just like this forum did not like mixing the MH case with Hannah Grahams at first because it really did confuse things. Also, if there is a jury that just doesn't work, or the DA did not present the case in a way to get optimal result, the other cases are still available to put the perp away. Usually the strongest case is tried first and if the sentence is such that it would be redundant to try other cases, it often isn't done for financial reasons. IMO, the MH case will be tried regardless, given the parents' involvement in the case. I can't see the Harringtons letting it go, unless JM gets the chair. Even with guaranteed life, they would likely want their daughter's case be tried and justice done for her. That is often an issue with perps have multiple cases.
I don't know how much of "slam dunk" the 2005 rape case would be. So much time has passed. I believe the victim is out of country and it is possible she will not come to testify, and if she does, how much impact would her testimony have? The DNA evidence is what would convict JM if there is a direct match, but I believe the victim would still have to be here for the trial, or is that not the case?
I think the DA gets to choose how to go within constrains of state law.