Chosen Solution
I am about to order a 1TB PCIe SSD and a 2TB SSD from OWC. My Stock 1TB Fusion drive is cloned and once I have upgraded my iMac with the new drives, booting from a usb stick with Mojave, will I be able to install Mojave on the PCIe SSD and have that as my boot drive and the 2TB SSD as a secondary data disk?
Yes! You’ll need to run Disk Utility first to format each drive. But before you take the system apart first break the Fusion Drive set Split Your Fusion Drive Apart I’m not sure why you are going with a SATA SSD drive given the fact you’ve got a better PCIe blade drive, I would go with a larger (and cheaper) SSHD which will get you better performance than a HD Apple uses Seagate 3.5” FireCuda SSHD.
