Yes you can create the set up as you desire. I would recommend creating 2 separate disk groups each with their own RAID configuration.
Disk Group Disk groups are composed of one or more hard drives
combined into a single data partition. You can think of disk groups as
the foundation on which one or more volumes or iSCSI LUN can be built.
Disk groups provide greater flexibility, allowing you to do the
following: Choose from various RAID types, depending on the number of
hard drives installed. Create a single, large volume, or create
several volumes with customizable storage capacity.
TO do this:
Add your disks to the machine
Install DSM
Open storage manager and navigate to DiskGroup
On there create the disk group by adding 2 drives and adding the level of RAID you require.
Then rinse and repeat for the other 2 drives.
On mine I have created 2 disk groups and added them as 2 separate SHR configurations check out the photo:
Separate RAID configs
This allows the setup as you have described. DSM is very powerful and would allow all sorts of configs.
HOWEVER -
I would recommend looking at something that Synology use called SHR-2/SHR. These are more efficient for space vs redundancy and easier to manage.
A handy calculator for RAID spaces etc:
https://www.synology.com/en-uk/support/RAID_calculator
I think your DR solution needs more thought though, if it's redundancy and backup you're looking for best practice dictates that you need a proper backup solution, look up bullet proof backups. So first local & then offsite (cloud, physical USB that is taken away etc)
Questions to answer are:
What happens if the synology box is stolen? Backups would go with it.
What if the site the synology box is destroyed / inaccessible?
Anyhow, let me know if this helps and if you need help setting this up more than happy to help offline too.
