No they can determine a phone's location even if it's not being used. And this is how they can sometimes determine burner phones have being used, even if the perpetrator disposes of the burner phone. So they track someone's actual phone to a location using cell tower triangulation and the report they get also states that there was another phone (burner phone) pinging from the same location.
So I'd be surprised if they used burner phones and LE don't have those numbers.
1. Very good observation, Zedzded, who is not a Violin nor a Cello nor a Viola player! (smile)
If both regular and burner phones are ON simultaneously, they will ping at the same locations at the same times.
If the regular and the burner are used intermittently and alternatively, they will ping along the same route.
If the regular is turned OFF and the burner is ON, it will ping at specific times and specific locations such that it could be deduced who is using it.
And a WhatsApp text, or any text from a phone including flip phone of the 1990s (not a smart phone), are traceable rather easily. Besides Cell Towers, there are other means of tracing and surveilling regular or burner phones' activities.
However, there are means of using regular and/or burner phones such that they are NOT traceable, but it would be lengthy and ... inappropriate ... to describe here.
2. IMHO, there are ample evidences to charge both Harvey and Wendi for second degree murder, which fetches 19 years
However, any defendant is innocent until proven guilty, which would make their possibly simultaneous indictments hard to prosecute. Hence, the sequential process adopted by the State, in a divide and conquer manner.
It has been long speculated by sleuths that Wendi could be "next in line" after Donna, given the potential perjury charges from her flip-flop on the stand, among multiple factual circumstances described in the dockets of Tuto Garcia, Kathy Magbanua, and Dr. Charlie. Besides, 19 year would be rather light for 46 year old State labeled un-indicted co-conspirator Wendi. Hence, the State might never indict Wendi until direct evidence such as Charlie's testimony availed. YT lawyers were exited by Magbanua's statements about Wendi but Magbanua got the info. over pillow talk from Charlie, which would be hearsay!
However, the "next in line" could be Harvey. As evidences transpire in the publicly available dockets' implications such as a) the birthday conversations to which Robert could potentially reveal, b) the registered communication at a restaurant with dear Charlie, c) driving the car used by Donna to deliver the wet and stapled blood money, d) attempted one way flight in Vietnam with Donna, and e) facilitating the alleged Donna's silly transaction to fabricate a witness and fictional statements by delivery of a painting piece as form of payment, to list just the most salient facts, could be enough for a potential second degree indictment. 19 years for a 80 year old individual is rather akin to life without parole.