In which case back to the original question (which was asked with the premise that one device was on ios, one on Android)
What happens if both are logged in at the same time? Do both get a warning that another device is logged in? Can both transact at the same time? If they do presumably some sort of security kicks in, perhaps logging one of them out? (or worse)
Is the security designed with the premise that only one device can be logged in at a time?
When I last tested this (about 4 years ago now), you could log in at the same time, see exactly the same thing, had no warning that you were logged in more than once on any of the 2 devices and could transact on either device. Intentionally doubling with a transaction was almost impossible though.
I can’t verify this as of today, but there have been countless security updates to the app in the last 4 years. I’ve access to plenty of Android devices but the 2 iOS devices we have are running a 
16-17yo account and a U16 account and I’ve no intention of messing them up.
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