Multiple seeds can be used, for example:
1. Seed #0, which is an internal seed that is not disclosed or stored. This must be brute forced when re-generating the private key, so it is quite weak. It is intended as some extra protection against an attack.
2. Seed #1 (stored in one location) which takes ~1 day to brute force if lost.
3. Seed #2 (stored in another location) which takes ~1 day to brute force if lost.
If the user has the passphrase, seed #1, and seed #2, all it takes is (say) 60 seconds to brute force the internal seed, and generate the correct privkey.
If the user loses either of the seeds, it takes 60 seconds + 1 day.
If the user loses both seeds, it takes 60 seconds + 1 day + 1 day.
I wold like to point out that your times are correct only if the user has a way to know that an individual seed has has been cracked. Otherwise, you must
multiply the number of attempts rather than add them.
Edit: Oh, I see that you have already arrived at that conclusion.