And I love that requirement. I do banking on my desktop and to confirm the transfers I get a push notification from a third-party application (ItsMe, so not a banking mobile app) with all the information I have entered.
I can confirm the transaction from a complete separate device while doing a second check if all details are correct.
The requirement per se is not the biggest problem. Implementation by different banks is. In my country I have several bank accounts.
One bank allows me to install mobile app on up to 5 smartphones, all I need is connect the smartphone to the Internet (e.g. through Wi-Fi).
Another bank allows me to have up to 3 smartphones, but identifies them by phone number, so it forces me to have 3 difrerent SIM cards
Yet another bank will only allow me to have mobile app only on one device. To activate on another device I need to receive SMS code, and if I lose my SIM card I need to show up at a branch in person.
I can confirm the transaction from a complete separate device while doing a second check if all details are correct.