I can tell a difference when I train on my road bike and then try to race XC. Does not seem like that should happen, but my N=1 shows that it does. Since my focus is XC, I train on that.
If cassette is the issue, change one over for the training.
If you do both, try changing between the bikes and see how each feels. I would just use each as I race on them so you can get used to the nuances of each. I would not FTP test for each.
Great job on increasing your FTP!