Depends on a number of things, I think. The Union of two disks in one increases the probability of data loss, so first make backup.
Also depends on what data is written where and in what order. The distribution of large data files on the SSD will be significantly faster than the SATA drive and the benefit will be lost.
If you use a home computer and not with the purpose of improving the reliability, you will get a car with a hybrid performance that is not as good as pure SSD in performance, but still better than the two 7200 RPM disk.
I would have done it himself? No, I would not reduce the reliability of my system and to increase the chances of data loss for fast disk performance.