It seems you mix up two concepts of OpenPGP: subkeys and certifications.
OpenPGP subkeys allow a primary key, which is used to create subkeys (technically, the subkeys are created independently but signed by the primary key using a binding signature). Such a set of keys (one primary key, an arbitrary number of subkeys) is considered a single trust unit in OpenPGP, ie. the subkeys are trusted if the primary key is trusted (trusting the subkey is not considered an additional step in the web of trust).
There is no such thing as "subkeys of subkeys", though -- only primary keys are allowed to bind subkeys. You may not construct multi-level subkey hierarchies.
On the other hand, there are certifications in-between primary keys. Using them, you can construct arbitrary trust networks (the web of trust) -- but each primary key is considered a single trust entity, and any other party would have to follow that trust chain and issue trust on each of the keys manually (in contrast to real subkeys, as soon as the primary key is considered valid, automatically the same applies to the subkeys).
Generally, consider a primary key to form an identity -- and (a single level of) subkeys can be used for key management and "keeping the primary key safe". You will have to use it from time to time -- creating and manipulating user IDs, subkeys and certifications on other keys is only allowed using the primary key. Certifications are used to provide trust statements among such identities. While you could of course use certifications to construct such a "two-level-subkey-hierarchy" technically, you are breaking the expected use of certifications and will have a hard time explaining third parties which key to trust (and use for what purpose).
