It’s not as simple as having two FTP’s, and so doing throws off a ton of TR’s calcs about fitness etc., namely TSS that are used to track fitness. Adding noise to what is the foundational number by which all the other algorithms are based introduces undesirable and unnecessary sources of error into TR’s currently clean processes, training plans, etc. IOW, it ain’t gonna happen.
I, presumably like you, have significant differences in the power that I can hold outside on my road bike and inside on my TT bike (covering the spread by using the extremes). Inside, I do all my TR workouts in aero on my TT bike, so I use the assessed FTP, in that modality, in that position. I also use Strava’s PMC (TP works better, but am unwilling to pay for a premium account) for outdoor rides and to generally track fitness, TSB, etc. over time.
You have to consider why these numbers are different - is it a cooling issue, positioning issue, I have a personal theory that sitting on an indoor trainer you are engaging far fewer stabilizer muscles (arms, core, etc.) than outside, and similarly are actually fighting the bike more indoors as the bike doesn’t move underneath you, but forces you to move around the bike.
This actually requires you to consider what FTP is… it’s your assessed FUNCTIONAL POWER - remove threshold from this, as that has physiological connotations which we’re not discussing here. Functional power is what you can do under those circumstances. In my case, I want to train in the extreme aero position, so that has a functional application. Whether it’s adaptation to the position (combination of flexibility and recruitment), that’s independent (correlation is not causation) from training your physiology.
However, my outdoor FTP is based on my MLSS, a physiological limiter, which is what I train to stimulate physiological adaptions. Moreover, it is off of this number that I track CTL and TSB, which should better represent physiological stress and how much recovery I need. Plus of course how I feel.
Training indoors on the ERG, even at lower power, elicits the same training benefit, even though it’s ~10% below what I can do outside. I know that’s hard for some people to grasp, but training lower that what you CAN DO, still elicits a significant performance result even though you’re not killing yourself to accomplish it. I would argue it’s even better for base training, because it doesn’t result in the same cortisol as maximum efforts, you subsequently sleep and recover better, and you can do more of it.
The rest is all for ego. Seriously. Power and w/kg are tools and metrics, and have nothing to do with performance. I have known guys with world tour power numbers that never get out out Cat 3 because they can’t perform in races. I would recommend instead of worrying about swapping back and forth to make sure you get all the internet points you’re entitled to, don’t worry so much about it. Enjoy the process, track your progress through performance improvement (FTP is NOT performance), and don’t worry about the other stuff.
People are so neurotic about this stuff that it becomes a stressor itself and ends up impairing or even negating progress that you’ve worked for.