Adding a LUKS Device - Avoid repeating password?

I’ve recently installed nethserver, using LUKS for / and swap. I find my cryptab looks like:

luks-<uuid1> UUID=<uuid1> none 
luks-<uuid2> UUID=<uuid2> none

And fstab:

/dev/mapper/luks-<uuid2>    /           ext4    defaults,x-systemd.device-timeout=0 1 1
UUID=<uuid3>                /boot       ext4    defaults        1 2
/dev/mapper/luks-<uuid1>    swap        swap    defaults,x-systemd.device-timeout=0 0 0

What perplexes me about this is that on bootup, I am only asked for my password once. I’m wondering how that happens, as I would like to add another LUKS encrypted hard drive for backups from the machines on the LAN. With Debian, I had to use keyscript=decrypt_keyctl, but some other magic is solving the problem here.