Unable to unlock multiple LUKS partitions with one passphrase

I'm using Ubuntu with a LVM over LUKS disk scheme. I have two SSDs, hence two LVM PVs are needed. They are both encrypted with LUKS, and have the same passphrase. Now on boot-up, I need to type the same passphrase twice. I want to tweak init to cache the passphrase (shortly) to unlock both partitions. I tried decrypt_derived, however it doesn't work:

device xxx uses the kernel keyring
Nothing to read on input.

How can I implement this? Thanks.