You don't say explicitly, but it sounds like you separated the LCD from both phones and swapped it over as shown in the Samsung Galaxy S7 Edge Display Assembly Replacement guide, is that correct? As opposed to moving the good motherboard to the other chassis, right? I'm not sure exactly how Samsung does their fingerprint sensor, but on iPhones they're paired to the motherboard, so I'm going with the idea that they are matched; original home button and original motherboard.
Going on that assumption, then you had to disconnect the home button cable in order to remove the motherboard. Have you double-checked that connector? Unplugged it, cleaned it, and checked for any bent or broken pins on the motherboard and the cable?
If that doesn't do the job, you may have to pull the screen back off and check the flex cable for any damage it might have sustained in the process of removing the display.
You've done all the reset/restart/update kind of software fixes first, though, right?
Let us know what you've tried already and give these ideas some thought and let us know what you come up with and we'll see what else we can figure out to help you.