But your spare phone still has wifi access? Is your spare phone on the latest iOS/Android version?Im still sticking to my more primitive way of using banking apps for my main accounts on a spare phone that has no communication apps (whatsapp etc) or data plan / sim card. Meanwhile, the banking app on my day to day phone is just a throw away account with a max of $150.
Would it be possible that someone who has backdoor access to your phone could wait until your spare phone connects to the internet (when you turn it on), then do the hijacking then?