Upgrading SSD and HDD to dual boot MacOS and Windows on each drive

Yes, you can use the OWC Aura Pro X2 in your system. Here’s the guide you’ll need to follow to put it in 27英寸 5K Retina 显示屏 iMac 固态硬盘SSD 更换

You will need to break the Fusion Drive set before you start and I also recommend you make a fresh TimeMachine backup as well to be safe.

Split Your Fusion Drive Apart

And no, you don’t want a dual SSD Fusion Drive, that would be slower than running each discreetly. The problem you are facing is the I/O speed of the interfaces (not the drives in this case) becomes the bottleneck! How to achieve the best transfer speeds with external drives, while this is more focused on external drives the I/O issues is the same internally as well. So what you can see here is if you want something faster than SATA you need to get to a faster I/O channel in this case Thunderbolt, but that in its self doesn’t overcome the limits of the discreet drive’s I/O and its technology. This then gets into RAID where you aggregate multiple drives to gain better throughput. So a single SATA or even PCIe/NVMe SSD’s limit is over come.

While that solves the drive config, you’ll still have issues with running macOS and Windows in the way you want.

I would recommend you re-think your config. I would still do a dual drive setup. I would leverage the PCIe/NVMe drive as a dual boot running BootCamp so you can run both macOS & Windows on it. The next aspect is where you store your apps & stuff! Here you want to limit the blade SSD to host your OS’s & Apps, leaving most of the drive empty! Then use your SATA SSD as your data store for both macOS & Windows. This is where I would partition the drive into two volumes one for macOS (GUID) and the other FAT32, exFAT or NTFS (depending on what your needs are under Windows).

If you really want fully isolated MacOS and Windows environments you could partition the blade SSD into two partitions. Frankly I don’t recommend this.