On a VM installed from the live installer, I ran update-initramfs -u after the install and initial reboot. Then I rebooted again, whereupon I was prompted for my disk encryption password a second time (for the other luks partition) after I had already entered it successfully.
As you can see from the screenshot, it even says that cryptsetup says it was set up successfully for one partition but yet I need to enter the password again for the second partition. I believe the password should only be applied to one partition, thus being prompted only once to unencrypt.
Additional reboots have the same behavior, being prompted to unencrypt both partitions.