I have this same question. I’m showing 2 drives but 1 is grayed out on the computer screen when I click on it nothing happens.
Be very careful. I also replaced my fusion drive with a 2T SSD and maxed the ram to 32gb but somehow I screwed up the microphone. I had to pay Apple 520.00 to fix it because they had to replace the whole back cover. Took about a week to get it back from Apple. Good luck!