I used to have a phone that handled two sim cards one was a UK number, used that for everything, the other was the regular 9 digit Portuguese number which was used by an app that sent text alerts for wildfires reported near my other house. It cannot send to 11 digit numbers (the app only allows 9)
Other than that, dual Sims for drug barons to halve the amount of phones they carry.
Dual Sims for home and office usage (turn the works one off after hours!)
One for your wife, the other for your lover, the list is endless really!
Just had a word with a friend who said this is OK as long as no names are mentioned!
I have a friend who suspected her husband of cheating, who had a dual sim phone, so she set up a new number in there and 'accidently' sent an innocuous
"I'm sorry I can't make tonight, I'll see you at the weekend x" to her husband's number, hastily followed by a "please ignore wrong number."
Well that's how it started.
He replied, saying he'd meet her if she'd been stood up, etc etc, things went from bad to worse, and she found out some horrid things about him and his thoughts.