How can someone get charged with two counts of first-degree if they only murdered one person?

Multiple counts of first degree murder can be charged based on alternative theories of murder. There is intentional murder, murder through actions that one knows will highly result in death, felony murder where victim dies during defendant's commission of a felony crime, and a few other variations on those themes. The facts dictate which murder counts to charge and how many. The number of counts is not limited to the number of victims.

It's not that the person is being charged with two murders. There are several different theories of law under which the defendant has been charged. That is not uncommon for many different crimes that can be charged under different theories.

That is entirely normal and happens in virtually every murder case. The murder statute defines several distinct mental states that can support a conviction for murder. The prosecutor usually charges every mental state that can potentially apply as a separate count. If the defendant is ultimately found guilty on more than one count arising out a single act, the counts are merged into one conviction for purposes of judgment and sentence..

Beyond all these excellent answers, the First degree Murder statute itself describes about six ways to make a death a murder. It is highly unusual for any Murder to be chargeable as just one of these methods. I would probably have charged the murder of which you speak in at least two and maybe three fashions.

FWIW, I know/knew all the players and it is a tragic, depressing story.

The concept of multiple counts being charged is based in the government's power to develop and prosecute, consistent with the statute, multiple theories of prosecution. This is also a strategy that the prosecution uses to weaken the potential theories of defense. More often than not the prosecution "overcharges" so as to set the tone for negotiation and to restrict the defense from using so-called technical defenses.
The prosecution and defense of criminal cases takes place in a complex environment of law, facts and strategies. The defense of any criminal matter is a multifarious endeavor that requires not just a knowledge of the law and a complete investigation of the facts but also a cognizance of the local prosecutions tactics.