I'd like to expand and clarify what Maximo wrote.
In either of the GEN failure modes that he mentioned (loss of A or loss of B), there will be no power to the airframe to keep the battery charged. If GEN B is lost, the engine continues to run on GEN A and the airframe gets no power. If GEN A is lost, the ECU will steal GEN B and again, the airframe gets no power. In either case the engine will run on the remaining GEN and the airframe will run on battery only.
As long as the Battery Master switch is closed, then everything that's downstream of the Battery Contactor continues to be powered by the battery. Your avionics will not know that anything has happened and will not switch to their own battery backups, as they'll still be powered by the ship's battery. The only way to force them to make the switch is to cut their normal source of power. I'm not familiar with the Sling; perhaps you have an Avionics Bus switch that could accomplish this.
As you and Maximo noted, if both charging systems fail, the engine will quit. You'll have to close the EMS Backup (Rotax calls this Backup Battery) switch to restart the engine, then it will continue to run only as long as the battery lasts.
With a lead-acid battery you'll see declining voltage that will give you some advance warning that the battery is getting critically low.
Not so with a lithium battery; they hold up their terminal voltage until very near end of charge, then voltage goes off a cliff. As you can see in the attached graph, you'll be lucky to see more than 0.5V difference between 40% and 80% discharged. Engine failure is likely to be very sudden as the battery's BMS cuts output to protect the cells against over-discharge.
This is likely the main reason why Rotax specifies a 16Ah battery for the 900-series iS engines. They have no idea which battery chemistry will be used, what avionics or other loads are installed in any particular aircraft, or how its electrical system is configured, so they specify a high capacity battery in hopes of getting you on the ground before it goes dead.
It's a good idea to develop an electrical load-shedding plan in the event that this happens. Again, I don't know what the Sling's electrical architecture looks like so I can't make any specific recommendations, but your goal should be to get as much load off of the battery as possible to maximize battery energy available to the engine. I would do this when the first charging system fails, just in case the second follows it.
Some manufacturers and amateur builders are using, e.g., EarthX ETX-680 batteries with these engines, despite the fact that, even when new, they fall well short of the Rotax specification. After three or four years on the firewall, how long will one of them -- or a similar capacity battery of any chemistry, for that matter -- run an ECU, ignition modules and high pressure fuel pump? Has the owner/pilot tested the battery and done the math? How about if there's no load-shedding plan and all of the avionics and lights are left on?
Granted, this is a pretty low-likelihood scenario, but it's probably worth thinking about before it happens.
