As a consequence of this you need to route channels across both interfaces over the DAW with the full RTL.
An alternative would be to make the "Digiface USB" to the main interface and to connect the UCX II via ADAT.
But then you can only route 8 channels over ADAT at single speed and only 4 at double speed.
Furthermore, you can't make good use of DURec, only for max 8 channels over ADAT.

If you're thinking about aggregation, you'll need to synchronize the interfaces via ADAT, as the Digiface USB has no word clock (WC). This uses up at least one ADAT port on both the UCX II and the Digiface USB for this purpose.

I don't know your exact requirements or target setup from this thread (how many devices you intend to add),
but generally speaking:

If you want to add a few devices and have the flexibility to route any input to any output, you may run out of ADAT I/O rather quickly.

It happens even faster:
a) at higher sample rates (e.g. double speed which halves the number of channels via ADAT), and
b) depending on what devices you connect.

Regarding b), some mic preamps don't have word clock or digital input for clock synchronization. Such a device needs to become the clock master, and you can only connect one such device in a setup.

What I want to point out is that this is an isolated question, and the answers may already help you. However, it's often more useful to know the whole picture to check if all requirements are met. For example, if all devices in the setup can be clock-synchronized and which device can or must be the clock master. Ideally, the main recording interface should be the clock master. That way, you only need to set the sample rate in the DAW, and the slaves will follow the master automatically.

If you want to expand with ADAT: the only interfaces with more than two ADAT ports are the RayDAT (PCIe, more ports, optional WC module) and Digiface USB (no WC).

Depending on budget and demand, it could also make sense to change the strategy and implement something that also supports MADI.

You mean something like this (constructed case):

PC--USB---Digiface USB[cs]<-----ADAT----------Device#2[cs]
|                                                                        |
|                                                                        | WC
|                                                                        |
+---USB---UCX II[cm]--------------ADAT-------->Device#1[cs]

UCX II is master
Device#1 is slave to UCX II via ADAT IN
Device#2 is slave to Device#1 via WC (both need WC or suitable digital out/in's)
Digiface USB is slave to Device#2 via ADAT IN

Ok, but then you have to manage the external device (Device#1) in two different TM FX instances.
The inputs in the Digiface USB
The outputs in the UCX II

In this case, not so beneficial that you can't directly route the inputs of external device
to your main interfaces (UCX II) outputs.

PC--USB---Digiface USB[cs]<-----ADAT---------+
|                                                                     \
|                                                                      \
|                                                                       \
+---USB---UCX II[cm]--------------ADAT-------->Device#1[cs]
                           \
                            +---Main Out

Then maybe better this way, so that e.g. the preamps or other ports of Device#1
can be monitored on the main interface UCX II.

PC--USB---Digiface USB[cm]-----ADAT--------->Device#1[cs]
|                                                                      /
|                                                                    /
|                                                                  /
+---USB---UCX II[cs]<-----------ADAT-------+
                           \
                            +---Main Out

But still settings for the external device need to be performed in two different TM FX instances.