> This assumes that these two persons will never need to use a smartphone at the same moment, which is a bit of a logistical puzzle.

In general no one wants to share anything with anyone, but when two people cannot afford a device individually, but it is within reach when they buy it together, time-sharing becomes a totally acceptable solution.

> Installing apps is the trivial part; isolating, or removing / reinstalling user data is much harder. An SD card could work maybe.

Checksums might overlap by quite a bit. No need to remove programs installed by both users. If the total installation of each user is 10 GB, but the installation diverges 300MB only, not a big deal in most cases.