LVM

LVM is greatly extensible and can be set up into Volume Groups.

You make an LVM drive array on the drives. Then you partition it in two partitions, 2TB each, for both drives that makes VG volume groups. so you have 4 volume ground A0 A1 B0 B1. You put a RAID 1 on A0 and B0 and a RAID 0 on A1 and B1. It's not really partition it's just volume groups. You format your two RAIDs, A is 2TB and B is 4TB. Nice If you are booting from them you have to lose space from the boot partition but it can come from the 4TB VGs. I use a small ssd for boot so no problem cool You can do it in the debian installer if you are installing fresh. I'll note this down Yeah LVM is recommended it's extensible. You can grow a RAID https://raid.wiki.kernel.org/index.php/Growing