> I know you cannot use both at the same time

Says who?

Reading the manual helps. Useful details for multi-interface operation are in there.

As you can see on driver download page both recording interfaces use the same ASIO driver.
So in your DAW you can access both recording interfaces via the RME Firewire ASIO driver.
You need to open a second instance of TotalMix FX.
Select in one instance your main interface, i.e. the UCX, and in the other the 800.
The only (little) restriction is, that you can't route channels between these 2 TM FX instances.
But all channels are useable by your DAW.

You only need to clock synch both devices, you could use for this purpose i.e. SPDIF coaxial, as both interfaces have SPDIF coaxial connectors.

Set i.e. the UCX as clock master (driver settings dialog: clock source "internal")
On the 800 set clock to come from the proper coaxial SPDIF input.
That should it be.

And you are still able to connect i.e. an 8-port preamp or AD/DA converter via ADAT to each of the 2 recording devices FF800 and UCX.