Double disk failure is improbable but not impossible.
The most impressive thing is that there seems to be no dataloss, almost whatsoever. Whatever the backup system is, it seems rock solid.
It's actually surprisingly common for failover hardware to fail shortly after the primary hardware. It's normally been exposed to similar conditions to what killed the primary and the strain of failing over pushes it over the edge.
For load balancing I would consider this very likely because both are equally loaded. But "failover" I would usually consider a scenario where a second server is purely in wait for the primary to fail, in which case it would be virtually unused. Like an active/passive scenario as someone mentioned below.
But perhaps I got my terminology mixed up. I'm not working with servers so much anymore.