I am sure that might be the case, but wasn't the point of a Fusion drive, for two reason, 1. cost of SSD drives, 2. wear and tear of the SSD drives. The spinning slower drive takes the brunt of the wear with commonly modified files, while fixed files, that don't change often go on the SSD.
The larger SSD drives are less expensive now, and the larger the drive, the longer the drives last, especially when using TRIM.
I still think with two SSD drives, it defeats the intent of an SSD, and would be more like a Redundant Array of Inter-spanned Disks. I suppose this could increase performance, if using a stripped array. I was under the impression though, that with SSD the seek time is so low, it also is not needed.
The only real benefit I could see would be to use two SSD as a Raid or fusion, would be for it to appear as one disk instead of two. Still, I would not do that.