Dual SIM is working fine with 2G, but not well with always on data. What's important to understand is that those dual SIM devices only have a single radio, that must be shared between both SIMs. And that's not part of the standard, so one has to play some tricks.
In the 2G era, the modem was mostly in idle mode. In this mode it sleep most of the time, and wakes up now and then (network configurable, around 1 sec. roughly) to listen for a paging opportunity. The network uses this time slot to send a paging signal to the device if it needs to wake up and reconnect because of a network initiated incoming call.
Because the active paging time is very small compared to the sleep duration of the paging cycle, it's pretty easy to handle two SIMs: register serially, and then the paging opportunity are unlikely to clash (and if they do one can play tricks until they don't).
Then if a call comes in on one SIM, connect for this SIM. You can't listen to the other SIM paging but it's unlikely you'll get two calls at the same time, and without answer another call will go to your voice mail, which is good.
That was dual active dual SIM at the time of 2G, and it was good for the users.
With 3G without data, it's about the same. But with modern 3G with mostly always on, and for sure with LTE with always on data, it's much more tricky. The modem will still go to idle a good part of the time even with always on data, but it'll wake-up much more often and at unpredictable times. Also, there are other low-power modes that can remove the need to go to idle for arbitrarily long durations (like connected DRX in LTE for example). It's just not safe to mux two active SIMs in this environment.
So in short: forget about dual active SIMs with 4G. You can still have dual SIM slots, with a user controlled switch and only one active at a time. But it's not as convenient.