Large (800 GB) Disk Image on Time Capsule as pseudo-NAS?
This is a bad idea on a Time capsule. It is possible, but it will be extremely slow to set up and very slow in use.

See Pondini's Time Capsule Q3

You can use Disk Utility to create a custom-size disk image (normal, not sparse) and specify it to be 800GB size, but you have to create that empty 800GB file locally first, then upload it to your Time Capsule. The Time Capsule's disk and network connections are not built for speed.

Even if you reserve 800GB of space on your Time Capsule, there is no quota to shared the remaining 1200GB fairly between your users. 

For example - One user could repeatedly fill their SSD with junk, back it up to the Time Machine target on the Time Capsule, delete the local junk and download fresh junk to the SSD. They could repeat this until the 1200GB space is used up. Only at that point would their oldest junk be automatically deleted from their Time Machine history. The other user would not be able to prompt any clear-out of the junk, they would only be able to lose their own oldest material.

Shorter answer: Time Capsule is great, but it is a backup device not a NAS: it is too slow and has no NAS functions. Don't try to use it like a proper NAS. It is handy for storing a few shared files like a very very basic NAS can, but that is very limited.