Your best options when you are already doing a fresh new install:
- make both drives one virtual one with LVM, then encrypt that virtual drive
But that's not related to BTRFS at all and can be done with any file system
- encrypt both drives with the same password, format them as BTRFS raid0 (again making it one big virtual drive), use the systemd hooks in your initramfs because then the first password you will provide on boot will unlock both drives (only if this fails on the first try you will be asked for additional passwords - also if you mistype your password once you will then have to unlock both by typing the password both; you only get one attempt to unlock everything with the same password).
If you can do this is mostly depending on the specific installer (I would assume only the first option works by default...). It's definitely possible in general but "I'm not technical enough" doesn't go well with all the details you have to figure out yourself when the installer doesn't do all the work.