HEY you, the one with such a contradictory nickname (and post! you said "luks for one partition", and then they talked about "two secret partitions")
IF "#secretA" is a FAT32 primary partition backup, and "#secretB" just a bunch of audio, text and compressed files, you can call me back and I'll personally explain to you (again) exactly what to do with them.
OTHERWISE, you should have listened jschiwal and fsmithred advices to get it through at installation time, they already gave you a positive answer to your question
(and no, they can not be encrypted on the fly by any debian distro installer: you'll need to backup that data away for restoring it into the the encrypted volumes once they are ready, and only this last part can be made at install)
OR you can try cryptsetup to encrypt'em "on the fly" under your running Debian, using gparted / fdisk / resize2fs (and a calculator) to free the necesary bytes (sectors) in the beggining of the partitions for luks initialization data. - acomplishing such a complicated task requires some further reading but might worth it -
TAKE a look at https://help.ubuntu.com/community/En...lesystemHowto7
(bottom, under "the LUKS conversion option")
Best luck wiht luks.
Don't bore us, get to the chorus.