I never tried it on a Sol-Ark specifically, but did you ever try a test to shut off or disable one inverter (on either master or slave, as two tests) to observe the behavior?
I have tested it on a pair of EG4 6500EX's for example, and I was required to go into the one standing inverter that is running, to disable the stacking mode (set back to SiG on Option 28 for single mode), so it is not looking for an remote clock signal to read sync signal waveform from.
I would think the Sol-Ark is no different, just go into Settings and to the Parallel tab, and untick the 'Parallel' check box, and save, then the inverter will operate in single mode again.
I have not seen an inverter that will auto-failover if one inverter is taken offline, but not to say that it could be coming down the pipe someday. Server clusters in the datacenter world do that all the time when they see a node heartbeat go down, they can switch states and failover automatically, but I don't think there are any inverters on the market that can do active to degraded failover automagically yet, but it would just be a setting change away for now.