The thing is that your FTP is not only depending on your peripheral muscles but to a large part on your cardiovascular system. Exercising with one leg only will really reduce the overall metabolic load for your body. But it’s a fun question and I vote for you to try it!
There definitely is research on single leg exercise like this old article, where single leg cycling only increased single-leg performance test results but not two-leg VO2max (in 7 people) (https://www.ncbi.nlm.nih.gov/pubmed/1150549).
People have tried single-leg exercise training for patients with lung disease. There is always the problem that you can’t really elicit training adaptations because their damaged lung is a bottleneck for performance. The idea was that by reducing the metabolic load by training one leg at a time, the cardiovascular system / lungs can handle it better while there is enough stress for the single leg to adapt. Results were not that great though, if I remember correctly.
I’m sure people are already using this after injuries but I’m sure others know more about that.