If BF is clock master then two ADAT cables are needed between BF and Mbox. In that case Mbox must be set as clock slave receiving clock via ADAT. (That is why second cable is needed.)

If Mbox is clock master only one ADAT will do and BF must be set as clock slave receiving clock via ADAT.

If you do not intend to have Mbox connected all the time, it is better to have BF as master. If you change sampling frequency in BF Mbox will automatically set itself accordingly.

Sorry, but this is not correct.
In this example, BBF Pro is the clock master and transfers audio and clock information via ADAT to the Mbox.
The 2nd cable is only needed to transfer audio data from Mbox to BBF Pro, just in case this is wanted/needed.
From clock distribution perspective, there is no need for the 2nd cable.

Babyface is clock master (driver settings: clock source = "internal").
Mbox is clock slave to BBF Pro and receives clock via optical.

PC---USB---BBF Pro(cm)---ADAT--------------->Mbox(cs)      <--- this connection transfers digital audio data and clock
                                   \---ADAT<---------------/                   <--- optional for transport of digital audio data from Mbox to BBF Pro
cm=clock master
cs=clock slave

See also my blog article about clock synchronization to get an intro / overview about the topic:
https://www.tonstudio-forum.de/blog/ent … ios-en-de/

In a digital setup, you require one clock master and only one is allowed.
The default setting for all interfaces is to use their clock as source, by this they are clock master.
In the RME driver settings, you will find as default "clock source = internal".
This means the internal clock will be used for AD and DA conversion.

For a clock slave, you need to configure the digital port as source, from where a valid clock signal can be received.

In your setup, I would recommend using the main interface as clock master.
With main interface, I mean the interface whose driver is loaded by audio applications.
Because then the sample rate is set by the application, the interface gets it automatically by the driver
and all connected clock saves will receive the clock automatically.
This is nice from an operational perspective.

The automatic switching of sample rate might have some limitations, though.
Switching between 44.1/48 kHz will work, but not between single, double, and quad speed
because with the ADAT protocol double and quad speed will be achieved by multiplexing 2 or 4 of the 8 channels for one audio stream and there is not something in the ADAT protocol header that tells the other side about this.
Auto detection of sample rate only works for SPDIF protocol (optical SPDIF, AES) or also MADI when using 96k frame format.

This is not a loop, as the digital cable does not connect to the same interface.
Let's better say the two interfaces are connected via ADAT in both directions.

There are two status messages in the Babyface Pro driver settings: lock and synch.
"Lock" means that the interface has locked on the sample rate of another interface.
"Sync" means that an interface has detected its clock on a digital input.
In a bidirectional wiring, only "sync" is acceptable.

If in your case, the BBF Pro is configured as clock master (clock source internal)
and you see in the driver settings "sync" on the ADAT input, then you configured the other side correctly as slave,
otherwise, the BBF Pro could not receive its own clock signal on the ADAT input.

As already mentioned, a single ADAT cable is fine to get the clock signal from clock master to clock slave.
The 2nd cable, you only need, if you want to transport audio from the Mbox inputs to the BBF Pro.

I would use such a setup as start point

+--------------------------------------------------------+
|                                                                        | optional USB connection to operate the device, if needed
|                                                                        |
PC---USB---BBF Pro(cm)---ADAT--------------->Mbox(cs)----Monitors / Headphones
                                   \---ADAT<---------------/             
cm=clock master
cs=clock slave

It appears that you want to perform the monitoring with the Mbox.
I propose using the BBF Pro FS as the main interface to have TotalMix FX as a flexible digital mixer on the PC side.
The driver of the main interface (BBF Pro FS) is being loaded by the applications.
Audio communication from/to Mbox only via ADAT through the BBF Pro FS.

As an option you may connect the Mbox via USB additionally, to operate / configure the unit if it has a control application.