The "dual SIM" use case is somewhat rare: - people willing to have both personal and work phone numbers in a single device - people willing to use two different carriers (example: one for voice and the other for data) It would have to deal with a number of extra issues: - temporarily disabling one SIM (probably you don't want "work" calls/texts outside work hours) - selecting SIM to use to send next SMS - allowing/disallowing data traffic from the other carrier when the selected one is unavailable - maybe also managing Contacts to avoid mixing work and family ones...
The only other reason to choose the F5122 over the F5121 is the internal memory (64 Gb vs 32 Gb). But if you use a cheap 64 Gb microSD to store all of your "large" (i.e. multimedia) data, you'll probably won't consider an issue having only 32Gb (Jolla1 was 16) inside...