By the way, so I take it that the idea of a two stage storage set up is not favourable as opposed to a single one.
That's not... necessarily true. There are advantages and disadvantages to both approaches.
Tiered storage allows the end user to upgrade their main storage, allows for simple external storage, gives devs consistent and reliable pinpoint accurate stats on drive performance, it has potential to be cheaper for large amounts of storage. It will require smarter software to manage data being moved around, it will be more complex, but if done right, it can be invisible to the user and have virtually the same kind of performance.
Single storage is simpler and more straight forward, but it's likely more expensive and will place limitations on user upgradeability (if you make it upgradeable, devs will have to use more relaxed perf stats) and external expansion (you either have to copy stuff to the internal SSD or implement a caching system that likely won't be as robust because it will be optional).