The biggest issue is resource limitations - multi-user systems require an abundance of resources that phones don't have.
What about the memory overhead of concurrent sessions?
Who gets to run background processes, and when are they terminated? Because neither CPU nor battery life exist in abundance.
What happens to incoming calls/e-mails/texts/notifications? (Especially for the guest account, where you don't have a second phone number for that account)
What happens to e.g. alarms set by user1 if user2 is logged in?
Which settings are shared, which aren't? And if your phone storage is encrypted, how do you handle shared settings? What about privacy? Can user1 e.g. record GPS signals even if user2 is logged in? If not, what about "Find my phone" features?
Sure, conceptually it's a solved issue. Practically, there are innumerable details to be figured out.
What about the memory overhead of concurrent sessions?
Who gets to run background processes, and when are they terminated? Because neither CPU nor battery life exist in abundance.
What happens to incoming calls/e-mails/texts/notifications? (Especially for the guest account, where you don't have a second phone number for that account)
What happens to e.g. alarms set by user1 if user2 is logged in?
Which settings are shared, which aren't? And if your phone storage is encrypted, how do you handle shared settings? What about privacy? Can user1 e.g. record GPS signals even if user2 is logged in? If not, what about "Find my phone" features?
Sure, conceptually it's a solved issue. Practically, there are innumerable details to be figured out.