I have the InsideRide E-Flex Kickr system, and I use a single FTP. I do ~90+% of my structured rides on the trainer 365 days a year, so no real reason to worry about the difference in the power I can put out inside vs. outside.
I know I can put out more power consistently outside, so I just use what I can do inside as my FTP, and bonus for when I’m riding outside - groups rides (or solo rides now), gran fondos, sportives, etc. - and I’m stronger.