The hardware is totally capable of having both network connections up at once. It could have a transition period where the data is sent over both networks for

The hardware is totally capable of having both network connections up at once. It could have a transition period where the data is sent over both networks for a few seconds until the transfer could be completed seamlessly. Sure, it would require app support, and server support, but if the OS supported it I'm pretty sure app and server support would happen. Alas - sadly it's currently impossible for an app to talk over both wifi and cellular at the same time on android - although it is possible for two different apps to each be talking over different networks at the same time. This functionality is used for the captive portal signin screen so you can be signing in to one network while still connected to another.