Definitely nothing wrong with that. People have spent way more outrageous amounts of money on other hobbies so just go for it.
If the expense is worth it to you, there is no reason not to. And you'll likely find other uses for the secondary phone too.
If you ever want to do more serious surveying, look into what people are doing with RTK antennas and their smartphones. There are definitely some interesting options for very serious centimeter-accurate surveying that can be done with just a smartphone and an external RTK antenna.