The two recording interfaces use different ASIO drivers, but you can load only one ASIO driver in your DAW. To work around this, you could use the third-party software ASIO4ALL to combine various audio drivers (ASIO and non-ASIO) and communicate with your DAW via the ASIO protocol. In the DAW, you load the ASIO4ALL driver, granting access to all recording interfaces (regardless of driver type) and even USB microphones. However, this undermines the ASIO driver concept, which typically allows direct, high-quality, low-latency access to recording hardware. By adding an extra driver layer with ASIO4ALL, you lose these advantages. This setup is unsupported by the manufacturer (RME) because third-party software is introduced into the audio flow, meaning the application no longer has direct access to the hardware, negating RME's high driver quality.

Even if you use multiple devices with the same RME ASIO driver, there are limitations. First, all devices must be clock-synchronized, either via Word Clock (which the Digiface USB lacks) or a digital "cross-connection" (ASIO, SPDIF, etc.). The DAW can access all recording interfaces since they use the same ASIO driver.

However, each device is controlled by its own instance of TotalMix FX, and there's no internal "backplane" in the driver to connect these interfaces. This means you can't route inputs or software playback channels from one interface to the outputs of another within TotalMix. Such routing between different interfaces can only be done through the DAW, resulting in significantly higher latency compared to near-real-time routing within a device's matrix.

Another option is to create a cross-connection between interfaces via ADAT I/O ports, which also handles clock synchronization. However, this is limited to routing only 8 channels via ADAT at single speed (44.1, 48 kHz) and just 4 at double speed (88.2, 96 kHz). This setup is also less convenient, as you need to route a selection of channels to the ADAT "crosslink" on one interface and correspondingly route these ADAT channels to outputs on the other interface.

TotalMix FX allows you to switch between multiple interfaces within a single window, keeping only one window open on your screen. Alternatively, you can open separate TM FX windows for each recording interface and arrange them on your desktop. When you save a TM FX workspace, it also saves the positions of all TM FX windows, which can be restored as needed.

ARC USB
The ARC USB should be activated for only one of the connected recording interfaces; otherwise, actions will be executed on all interfaces simultaneously, which usually leads to undesired results. You cannot instruct the ARC USB to perform actions for specific interfaces.
Therefore, activate the ARC USB only for the interface to which monitors and headphones are connected—the "main interface."

In contrast to that Apples "aggregate device"

With Apple, it's different. There's no direct hardware access like with ASIO; all audio flows go through the Apple sound system. Apple’s OS allows the creation of aggregate devices, a solution supported by RME because it operates within the Apple sound system, through which all audio must pass. This is unlike ASIO4ALL, which is an added "fake" ASIO layer that only mimics the ASIO protocol without providing direct hardware access.

Here are your options:

1. Buy the Digiface USB, synchronize it with the Multiface via ADAT, and route through the DAW. While you could route individual channels via an ADAT cross-connection using ASIO4ALL, I don't recommend this unsupported setup. It would be a makeshift solution, and you wouldn't be leveraging RME's strengths, like driver stability and low latency.

2. Get a second or third Multiface, synchronize them via Word Clock or an ADAT cross-link. With one ASIO driver, the DAW can access all connected Multifaces. RME tests up to three Multifaces; more might work depending on your PC and whether it can handle the channel count over PCI.

3. The best solution, and my recommendation, is to get a single RME recording interface that provides the needed number of I/O ports, allowing near-real-time routing of any input or software playback channel to any output within the interface.

You might need to consider the 802 FS, UFX II, or UFX III with MADI. I'm speculating here, as your exact connectivity needs haven't been discussed in this thread.

Maybe another possibility, RayDAT (PCIe) based

Another option that provides plenty of ADAT I/O for connecting other devices is to install one or two RayDAT cards in your computer. Clock synchronization can be achieved via Word Clock (WC), saving the cost of optional WC modules by using internal connectors between the cards inside the PC (see RayDAT manual, via X400 Sync In and X401 Sync Out).
This setup ensures clock synchronization. Alternatively, you could use cross-connections, with the card offering ADAT, SPDIF (coax), and AES options.

The advantage of this solution is that it provides two high-quality, fully digital PCIe cards with extensive connectivity. A single RayDAT PCIe card offers: 4x ADAT (one switchable to optical SPDIF), SPDIF (coax), AES, and 2x MIDI. There are no analog ports, so you would need to connect external converters or preamps via ADAT, either from RME or other manufacturers if budget constraints apply. A very budget friendly solution is to use ADA8200 which can be used as Mic Preamp and AD/DA converter (limited to single speed, not more than 48 kHz is possible). But you could also connect other devices like recording interfaces in stand-alone mode that can load their config and the needed routing during power-on. Alternatively install their drivers to be able to at least operate them with their management software. You wouldn't use the driver in the DAW, only for operating those devices.

At a later point the RayDATs would also allow you, to integrate an RME reference converter for high-quality monitoring. The ADI-2 Pro (or ADI-2/4 Pro SE) can be connected via AES, leaving ADAT ports free for other external devices.

But remember, as I mentioned already, every RayDAT is kind of "isolated" (no internal "backplane" between multiple cards).
You have to work with two TotalMix FX "windows" / "instances".
To route channels from one to the other card you need either to route through the DAW with higher latency or though a digital "cross-link" with near-zero latency.
The RayDAT also provides internal connectors to create such a crosslink, see manual.