In comparison to have to iPhones, the main issue for me is that you can not use both SIM cards for iMessage and FaceTime
Today I frequently send iMessages from both iPhones so only being able to send iMessages from one number is a big problem for me
Also for me both iPhones have different apps, email+calendar accounts, contacts, etc
They work independently from each other with no shared/synced data between the iPhones (other than iMessage communication for file transfer).
For this to work properly for me the iPhone should have multiple accounts, similar to accounts on Mac