A DAW can't handle more than one ASIO driver.

Use UCX as primary device and clock master, connected to the PC.

- I got the UCX running via USB already, will I need to change any specific setting at the device to run it as a master?

Connect motu via ADAT, preferrably UCX as clock master (Steadyclock is superior).

- How do I connect it via ADAT and what does 'Steadylock is superior' mean?

Configure routing and route the channels via ADAT IN/OUT as you like.

- I also am not really able to follow you on this one, is that a software mixer related task?

If Motu supports standalone mode, you have to configure routing only once.

- I think I can run the MOTU as a standalone device. It's a soundcard I have used before I got the RME and figured I shall put it at use again to have more ins and outs available... I found the following info on their site about this standalone topic if that is what you mean?:

Using the 828mkII & 896HD as standalone converters
Using the Firewire Cuemix Console, you can set the 828mkII or 896HD to be a standalone format converter.
The 828mkII and 896HD can pass through D/A or A/D audio even in standalone mode. Using the Firewire Cuemix Console, you can set up your 4 stereo busses to accommodate your conversion (the 828mkII can set up using the front panel's knobs as well).
For Digital to Analog, use Firewire Cuemix Console to:
Set Bus 1 to output to Analog 1-2
Set Bus 2 to output to Analog 3-4
Set Bus 3 to output to Analog 5-6
Set Bus 4 to output to Analog 7-8
On Bus 1, mute all channels except ADAT 1-2 and make sure the faders are up and the channels are hard-panned left and right
On Bus 2, mute all channels except ADAT 3-4 and make sure the faders are up and the channels are hard-panned left and right
On Bus 3, mute all channels except ADAT 5-6 and make sure the faders are up and the channels are hard-panned left and right
On Bus 4, mute all channels except ADAT 7-8 and make sure the faders are up and the channels are hard-panned left and right
This will route every incoming ADAT channel to a discrete analog output.
For Analog to Digital, do the same but switch your outputs to ADAT and activate the analog channels.

Otherwise you need to keep the MOTU driver/software installed to be able to reactivate the routing on each PC reboot.

- does this mean I will not need to install the motu drivers?