I've also spent a lot of time researching this mainly because I can't find the source citing MG as wondering why the daughter did not take her device on the road trip, alleging she saw it paired in BM's truck. (I'm patiently waiting for one of our absent WS members that initially posted this entry to join us again).
I'm fairly confident that the second device cited in the AA as
associated with BM's iPhone with an SMS [text] at 3:43 PM on 5/8/20 is another Apple device capable of SMS [texting] which could be an iPhone, iPad, iPod, Apple Watch, or MAC.
You can only have one User ID per Apple device by design (single users only). However, the
User ID noted as being separate from the
Primary User ID (i.e., Apple ID) could mean the
primary email associated with the Apple ID /iCloud is different than the
email for App Store/iTunes.
In the AA, I think it's clear that LE knows the phone number associated with that second device (it's redacted).
We also know that SM intentionally separated her Apple electronics from the family, and at a minimum -- NOT sharing the same iCloud but I also think that SM probably wanted MM2's phone associated with one of the adult accounts where MM2 could also have her own User ID for the App Store/iTunes. On the other hand, I could definitely see BM and MM1 sharing App Store/iTunes which would also appear as a separate device from the Primary User ID (primary email different) but still associated with BM's iPhone.
There are actually some very specific instructions at Apple support to unlink your phone/device from another phone including how to tell if your phone is linked to another phone/device, and delete devices connected to your Apple ID.
I think I hate this stuff...