The UFX uses a different ASIO driver than the UFX II.
An application (DAW, Music player, …) can only load one audio driver at a time.
The minimum ASIO buffersize of the UFX II's ASIO driver is 32 @single speed, for the UFX 48 samples.
Using ASIO4ALL I cannot recommend, as this is a not supported setup. By this, the application has no direct access to the audio HW anymore, everything has to go to an additional ASIO layer on top through ASIO4ALL.

If you operated both units in parallel, then you will have additionally an offset when recording using channels of both units because they use different converters, the UFX II has quicker converters.

Next thing, each interface would have a TM FX instance with no routing possibility, there is not something like a backplane between the two interfaces.
Therefore, you have only two possibilities:
a) to route in the DAW (where all monitoring flows would have the full RTL (round trip latency) through USB)
b) to use one or two ADAT I/O ports to route up to 16ch between the interfaces at single speed
For clock synchronization purposes you would need anyway a digital connection (ADAT/SPDIF/AES) or by using WC (word clock).

So, a better option might be to use the UFX II with the MADIface driver as main interface and to connect the UFX in standalone mode through ADAT (either using 1 or 2 ADAT connections IN and OUT to get 8 / 16 ch @single speed).
You can install the ASIO driver for the UFX and connect it through USB as well to keep it operational and to have more freedom which channels to route from or to where on the device.

The ideal solution is to get a bigger recording interface with more channels and possibilities to expand.

Like for example getting the successor of an UFX+ or to get a UFX+ from used market.
If you require more channels, then you can better expand by using MADI.
MADI has also some advantages like e.g.
- higher channel count (64 at single speed)
- to be able to connect multiple external devices serial one after the other and back to the recording interface and
- to support longer cable length using multimode fiber (OM3, OM4), up to 2 km (!) between each of the devices.

OM3/4 multimode patch cables are available up to ~50 m. Compared to this, ADAT is limited to 8ch at single speed and a max cable length of 10 m according to standards (but usually works up to ~15 m).

This makes it easier to distribute devices across different rooms: control room, recording room1, recording room2.