SCR fail safe protection
You can't make something totally fail-safe. However, you can make failure less likely, and you can make that failure less problematic.

First, make it less problematic. If you have two switches, one on each side of the load, then a failure of one could leave the load connected to the 'wrong' mains pole. That can be mitigated by putting the two SCRs in series in your 'higher reliability' switch, and putting those in the pole you want to switch.

If the failure of any given switch is independent of that of the others (and this may be true for some mechanisms, like a comsic ray strike, and not true for others, like an overvoltage spike, overtemperature, overcurrent or old age), then putting multiple units in series will make failure less likely. If one switch has a failure in 1 in 1000 operations, then 2 switches will be 1 in 1 million, and 3 switches will be i in 10^9. 

As you see from my list of what failures would be random, and what related, it's probably better to protect the SCRs from fault conditions, than to simply series up a number of them.