Easy enough to say "Gee...these 2 phones are always together or nearby when activated" or "this phone shuts off right before this one powers up".
Although, I suspect there are a few other ways to determine identity easier. Such as tracking the device identifier and then looking up nearby public facing cameras.
There's a nice article about him, he is a full-time train engineer who drives train shipments all around the west coast and while he was traveling he got curious about all the plants he would see from the train so he started going to the libraries on his breaks from work to learn about plants.
https://www.outsideonline.com/culture/books-media/botany-joe...
Interesting chilean high-elevation rare carrot: https://www.youtube.com/watch?v=UdodZcrFIPM&t=2s
old growth redwoods: https://www.youtube.com/watch?v=Xbma869jMQY&t=4s
Sidenote: I like that he tattooed his finger with measurement lines so he can use his fingers in the pictures as reference for size.