You have described a deterministic wallet. I think Casascius came up with this in 2011.

I think a simple example works as follows - start with secret exponent sha256(seed:x), with x being some arbitrary number (ie 0) and then incrementing for each new key pair. "seed" is the secret.

In your case, you are using the addition of two private keys as the seed. You always need to keep at least two adjacent key pairs to keep the chain going. Exploiting any two adjacent keys would reveal all subsequent keys. Such a scenario may be more vulnerable to attack or loss as private keys are likely to be contained within bitcoind wallets (which may or may not be connected to the Internet). Two "secrets" are also required to be remembered.

A traditional deterministic wallet seed is not stored within a bitcoind wallet.