Prevent macOS Sierra from prompting for the password for other encrypted volume If dual boot is used to run a second macOS installation off a different volume on the same hard disk, and that volume is encrypted, booting the first macOS will prompt for the encryption password for the second volume on each boot.  However, there is no need or desire to mount the second macOS volume from the first macOS. How can this prompting for the encryption password be disabled?

The solution needs have been verified working on macOS 10.12 Sierra specifically.
You can edit /etc/fstab if you don't want a partition to mount on startup. I haven't tried personally but there's a writeup at Mac OS X Hints.

In short, get the UUID of the partition and add something like:

UUID=F0E430C1-5558-3BB3-9FA9-6904B663FEEA  none  hfs  rw,noauto