It's my understanding, as I said before, (and I could be wrong), the FBI doesn't investigate murders, even serial killers.
Also, 2 murders does not a serial killer make. I believe the criterion is 3 or more with some aspect or aspects of each one identical in all three. Once they have established that the murders are in fact the work of a serial killer, they may assemble a team of investigators from every jurisdiction to coordinate and distribute information as it comes in. And if your killer is killing each person in a different state, it may take some time for the police to realize he is indeed a serial killer unless they check VICAP as I suggested in my earlier post. In that case, they would only find crimes that matched theirs in some way or another. The more detailed information they can provide VICAP, the more accurate returns they will get.
I've checked several reliable websites regarding the FBI, and nowhere on any of them does it say the FBI investigates serial killers. They primarily investigate crimes like terrorism, organized crime, and threats to national security.
Also, contrary to what you see on TV, the FBI does not ride into town and hijack cases from local police. They will assist if asked, but they are too busy to get directly involved in murders of any kind.