SSD Change of two Macs

Not sure I follow what you are trying to do here. How about reviewing this great guide that explains the different Apple custom SSD’s The Ultimate Guide to Apple’s Proprietary SSDs

You also need to better understand what a Fusion Drive is. It’s a standard HDD SATA drive and uses a second drive (PCIe blade SSD) as a cache drive for the HDD. This second drive is not overly large. There is a point of no benefit when you get too large a SSD drive for the cache drive. Depending on the size of the HDD a 128 GB is largest that makes sense.

Here’s a bit more: Mac Fusion Drive vs SSD vs Hard Drive

Once you get past 128 GB a proper dual drive system makes better sense. As an example I often setup a 500 GB or 1TB SSD as the boot and application drive with a 1, 2 or 3 TB HDD setup as the data drive for audio and video students and pro’s.

Here’s a bit more: How to set up a Mac and macOS to take advantage of an SSD and external storage setup or second internal drive