how do you ensure this? afaik ext4 scatters its allocations across the partition, so it's only a matter of time before an allocation for a syslog entry wipes your .ssh/id_rsa
Colocating both roots on the same partition spells disaster, you will absolutely clobber data when the outer system boots. I have detailed in another post that the correct way is to put the sensitive root inside a largely empty decoy data partition, and no longer write there (but you can update timestamps in place).
Needless to say, using these schemes for SSD drives requires special consideration in regard to trimming.
