Here is a topic I started a number of years ago.
It’s a PITA.
My ultimate solution was to use my Stages for everything. I use power from the Stages indoors and out. I’ve configured everything to ignore the power reported by the trainer. I even traded in that old Tacx on a used Kickr and found that it reads low by 20 watts across the board.
My conclusion was that you really can’t trust any of them to be “accurate” so just pick one and train to that power meter. Creating zones and training to one power meter works perfectly even if it’s off by 5, 10, or 20 watts because it’s all relative.
If you need to use two power meters then you have to do the comparison dance and maybe calibrate them to each other if even possible. And if they don’t agree you may need to buy a different power meter. I believe there is a pedal power meter that can be adjusted to match another power meter.