How many analog I/Os do you need in total? How many analog inputs and outputs? Mic / Instr inputs?

And yes, you could use UCX II and BBF Pro FS in the following way.

UCX II and BBF Pro FS use the same ASIO driver, so any application that loads the ASIO driver can access both interfaces.
You just have to make sure that they are clock synchronous (e.g., via ADAT) and use the same ASIO buffer size for both in the driver settings.
Otherwise, you will get an offset in, e.g., recorded tracks.

The converter latency of both interfaces is almost the same: 5/7 vs. 5/6 samples in/out. So only a negligible offset is to be expected. However, stereo devices should only be connected to one or the other at a time.

The only minor drawback is that you would have to work with two isolated TotalMix FX instances, one for the UCX II and one for the BBF Pro. You would have to route in the DAW if you wanted to send audio from an input on the BBF Pro to an output on the UCX II.

To get around this, however, you could do something different and connect the BBF Pro FS to the UCX II via ADAT and run it in standalone mode.
Then you can route the analog channels of the BBF Pro FS via ADAT and make them available on the UCX II.
Then there would also be only one TM FX instance where you can route any input and SW playback channel to any output of the UCX II.

In both scenarios, one of the two devices needs to be clock master. The clock slave can get clock through one of the digital links (word clock is not an option for the BBF Pro FS). You could connect a TOSLINK cable from UCX II ADAT OUT to BBF Pro FS (ADAT IN) for clock synchronization.

@waedi: as a side note … even though it turned out that you need more analog I/O and assuming an equal number of inputs and outputs. Connecting a 12Mic to a recording interface with only 1 ADAT input is a bad idea. You could only use 8 out of 12 channels. Too much money wasted if you can't use 4 of 12 ports at single speed and only in single speed.

As an alternative, get an UFX II or the successor of the UFX+ if your budget allows.
Ok, I know, it's not a small form factor, but tbh .. Better one device with many ports and options (possibilities!) and one normal power cable compared to having two little devices with external power adapters. I know BBF Pro FS can be operated USB bus powered, but I, personally, would recommend using a PSU to have a good power delivery.
With the UFX II you would automatically get much more compared to the combination of UCX II and BBF Pro.
Furthermore, the MADIface driver supports the same very low ASIO buffersize of 32 samples like PCIe-based products.
You get two equal headphone outputs with much more power.
Four excellent Mic inputs which can also be used as line and instr input.
Regarding Instr inputs, the UFX II has a wider gain range for instr inputs compared to UCX II.
My advice, instead of spending a lot of money for combining two products, I would buy the better product in one standard form factor.
You have also more possibilities to expand later, 2x ADAT I/O to connect more preamps or AD/DA converter
and 1x AES I/O to connect, e.g., an ADI-2 Pro FS if you want the quality and unique features of this unit.
So, you buy now and can keep the UFX II for a very long time and expand it further if needed.

Selling an RME interface is relatively easy and currently there is a high demand on interfaces and some shops even increased the prices. For a current interface in excellent condition, you should get around 75% of street price.

Hi ramses,

Thanks for all that great info!

I think you misunderstood, I need the additional ins and outs that the UCX II would provide over the Babyface right now, and that's why I'm upgrading to the UCX II. I'll never need all the functionality and channels of something like the UFX in what I do.

I'm curious about what you said regarding the MADIface driver and the UFX II. I thought I'd read that the UFX doesn't use MADI. I thought it was a USB interface. I'm not even sure what MADI is (I'd assumed it was a connection protocol like ADAT).

One other question: I'd read in the RME manual that it's not ideal to run system audio (meaning Windows default system sounds, web playback, etc.) through the same interface you're using for recording. I haven't had any noticeable issues using the Babyface in this way. Do you use a different interface for your computer's system sounds?

Ok, I see.

I also have Windows system sounds off. Is it only the system sounds that can cause issues with recording? I don't think I've noticed any problems.

I have had this exact setup for a little while. You can use 2 ADAT cables to connect the two interfaces. They both have 8 ADAT channels, so one's inputs are the other's outputs.

TotalMixFX will detect both interfaces and present you with two mix windows, one for each. So if you want to use the Babyface's inputs with the UCXII you route the Babyface inputs to its ADAT outputs 1/2 (in the TotalMixFX window for the Babyface). Then you switch to the TotalMixFX window for the UCXII and you should see incoming signal on the ADAT inputs.

Hope this helps.