I am not sure whether you are aware of this possibility to use both interfaces
and to expand your UCX by i.e. 8 additional line IN and OUT channels via ADAT.

For this
- keep both drivers installed
- connect UCX and Multiface to your PC
- connect UCX and Multiface by 2 ADAT cables (UCX ADAT OUT to MF ADAT IN and vice versa)

Clock settings in the RME driver settings dialog:
- UCX: set clock source to "internal" (Master)
- Multiface: set clock source to "ADAT" (Slave)
If you cabled correctly then you should see in the RME driver settings dialog the ADAT status changing to "sync".
Then the Multiface is clock synchronized to the UCX.

If only 1 TotalMix FX window opened, then you can either work with this one window and change between both devices by clicking to the upper right where the device name is shown. Pop-Up window openes where you can select, which device you want to maintain ...
Alternatively you can open a new TotalMix window, select the 2nd device, arrange the 2 TM windows on your screen, so that you can configure them both.

For the Multiface you could then route i.e.
- the 8 analog inputs to ADAT 1..8, so that they become available for the UCX as ADAT1..8 IN ...
- the 8 ADAT inputs to analog 1..8 out, so that they become available for the UCX as ADAT1..8 OUT ...
Save this i.e. in Snapshot1 of the TotalMix Instance for the Multiface and rename it i.e. "Lines4UCX".

The Multiface works now in kind of "standalone" operation modus and has a fixed routing which expands your UCX by having 8 more analog inputs and outputs.

What you should do now
- save snapshot and workspace
- assign this new setup to a workspace quick select slot under the name "UCX and Multiface +8 Analog IN and OUT".

I recommend the following TM FX Preferences (F2)
- Snapshots -> (  ) Do not load Main volume/balance
- Devuce Handling -> (X)  Always init DSP devices with TotalMix FX settings

By this you always initialize both devices with the configuration saved on your PC.
Especially the volume has then always defined values as saved in your snapshots not to blow your ears.

Voila, a nice solution to you ...

In the DAW you only work with the ASIO driver of the UCX.
The ADAT1..8 IN and OUT channels make now available the Analog 1..8 ports of your Multiface.

If you say that you need ADAT on the UCX for a Preamp .... then you can do 
But as preamps only rarely support 2 or more additional analog outputs (with a few exceptions)
then you can still use the Multiface to provide 8 additional analog outputs.

But then you require Wordclock cabling between the three devices,
as your Preamp only can get clock by an incoming ADAT signal....

Depends what you require ...
The UCX offers some nice features and gives you more possibilities to connect something.

Its also a question whats better for your workspace.
Some inputs and outputs that you will  require are located on the left side of the BBF Pro, i.e.:
- USB
- Power
- MIDI
- ADAT IN
- ADAT OUT

If you are a righthander, then the BBF Pro will be placed much likely right beside keyboard/Mouse.
And there it could be a little challenging to have so many cables lying around there.
I personally would regard this as disturbing and could only live with it when it is absolute priority
to combine recording interface and monitor controller in the most compact unit possible,
i.e. if you travel a lot ..

I personally would for this reason plan otherwise and get
- UCX (like you intended to do)
and IF you really like/want a monitor controller
- get ARC USB additionally

Then you can place the UCX and Multiface somewhere at the edge of your desk, near the screen
or at a desk beside your workplace. By this you have not so many cables (f)lying around over your desk.

ARC USB can be connected easily by USB to the computer, TM FX will do the communication towards the UCX.
In the 2nd TM FX instance for the Multiface you need to set under
- ARC & key commands TAB: "ARC USB Settings"
(x) disable PC connected ARC USB
To ensure that the ARC USB only works with the main recording device the UCX.

The BBF Pro is a phantastic compact and powerful unit, but if you do not require the very small form factor to combine monitor controller and recording interface, then I would take the UCX.

UCX vs BBF Pro:
- USB and Firewire (if you have USB issues and can cure by using Fw400)
- Or if you are on a MAC with thunderbolt and want to keep USB ports free...  then you could i.e. use a Thunderbolt to Firewire Adapter to connect UCX
- Nearly same low latency AD/DA converters: UCX 14/7 vs BBFP 12/7 samples latency
- UCX supports word clock
- UCX has the complete FX chip implemented, BBFP only partial, some things are rendered on the PC (dynamics, reverbs)
- UCX has autoset, BBFP has not: autoset is nice for levelling signals, either beforte, sometimes also during a record
- UXC has 6 standalone profiles, which can easily be recalled at the device

BBFP is the newer device, so some advantages i.e. in terms of SNR and potentially more up to date headphone amp design, at least it has a lower Output impedance.

You can watch the differences here in the Excel / PDF of my blog article about RME interfaces:
https://www.tonstudio-forum.de/blog/ind … B-MADIfac/

https://www.tonstudio-forum.de/index.ph … 017-12-pdf
https://www.tonstudio-forum.de/index.ph … 17-12-xlsx