If you have the actual probabilities, which include cascade failures and one of the systems is a safety backup system not in normal use then multiplication is reasonably though not completely accurate. Though, you do need to consider the failure of the safety system when the normal system is operating correctly, but that's also much easier to avoid so likely very very low.
Cascade failures are a far more complex situation and thus harder to predict. So again, it depends on what the failure rate is describing not just what the number is.
If you have the actual probabilities, which include cascade failures and one of the systems is a safety backup system not in normal use then multiplication is reasonably though not completely accurate. Though, you do need to consider the failure of the safety system when the normal system is operating correctly, but that's also much easier to avoid so likely very very low.
Cascade failures are a far more complex situation and thus harder to predict. So again, it depends on what the failure rate is describing not just what the number is.