It really depends on whether you are trying to achieve the highest MTBF (which can be applied to each component such as an SSR) or the highest equipment reliability which involves component redundancy used in a design.
The easiest option I can see is to have two SSRs in series, and do a check on startup to check that turning off either will interrupt the current, and display an error as well as refuse to operate.
This is a great strategy overall for high reliability, but is dependent on the absolute stress for each series component. In your case where a heater is involved you need to ensure you lower the stress on BOTH SSR's or else you compromise the achievable reliability.
A heater typically has a significant power up stress profile, it draws much more current at turn-on and until it heats up (3-6 times rated current is not unusual). Since your strategy is to have two in series to increases redundancy both carry the surge current and therefore are exposed to the same stress rating at startup.
To reduce surge current stress, you need to soft start the heater (phase control).
To reduce voltage stress you select SSRs with a higher voltage rating and put surge suppressors across the mains input.
To reduce thermal stress you need good cooling. Hot components have much reduced MTBF rates.
A good place to start is here with Crydoms reliability report.
Since SSRs (Triac and SCR based) typically have quite large voltage drops they can become quite hot and as the graph below shows even 65degC case temperature can significantly reduce the MTBF.