Hi per.

Welcome to the RME forum. I didn't watch the video, but it's always the same, regardless of what the video tells you.

There are basically 2 possibilities and this forum had already many threads regarding this topic, so you can use forum search (or extended forum search which gives you more options) or by using google search “search string(s) site:forum.rme-audio.de”.

Here is a summary for you what options you have and what the advantages and disadvantages are of each of the solutions.

As you need the double number of channels, you need to use option A).

A) connect both through USB or Firewire
    clock synch through one of the two digital ports: ADAT (optical) or coax. SPDIF
    ensure to use the same (ASIO-) buffer size in the RME driver settings
    Advantages:
      — double number of ports
      — full latency compensation for analog ports on both devices
      — number of analog ports stays the same regardless of the sample rate being used
    Disadvantage:
      — 2 separate TM FX instances for each of the two UCX. Therefore, you can't route channels across the two devices
          via TM FX. So, you need to route in the DAW with the full round trip latency through USB/FW.

B) connect 2nd UCX through ADAT, this gives you up to 8ch at single speed (44.1/48 kHz).
    Once you configured the routing of channels on the 2nd device, you can operate it either in stand-alone mode
    or keep it connected through USB/FW so that it is easier manageable
    (to, e.g., adjust Mic gain and such things through TM FX).
    Advantage: (only) 8 more channels through ADAT IN and OUT, all channels can be routed in one TM FX instance.
    Disadvantages:
    — no latency compensation in the DAW for analog ports behind ADAT port, you need to adjust it in the DAW
    — if you should use higher sample rates, then the number of channels over ADAT decreases to 4 and 2
        depending on whether you use double- (88.2/96 kHz) or quad-speed (176.4/192 kHz).

One device has to be clock master, there can only be one clock master in such a setup.
In the RME driver settings, the clock master has the setting: clock source “internal”.
On the clock slaves, you need to configure the digital port from which they get the clock signal (ADAT).
Word Clock cabling has no better quality and is not required, keep it simple, saves costs, better use ADAT/SPDIF for clock synchronization, if possible.

What do you mean by UCX 2, UCX II? Please stick to official product names to avoid (little) confusion. ;-)

If you mean UCX II, then you can connect both through USB, as both units use—under Windows—the same USB ASIO driver.
I am not 100% sure, whether latency compensation in your DAW will work fully and recognizes each unit individually because UCX II has a little faster converter.

Check manuals under “Latency and monitoring”

UCX, ch 39.2   : https://www.rme-audio.de/downloads/fface_ucx_e.pdf
UCX II, ch 40.2: https://www.rme-audio.de/downloads/fface_ucx2_e.pdf

Converter latency @single speed (44.1/48 kHz):
UCX    : 14 / 7 samples (AD/DA)
UCX II:   5 / 6 samples (AD/DA)