"Method 3" .. what video are you referring to ? There are plenty of videos.

When connecting multiple devices be simply aware of, that you can't mix channels across units.
The only think that you can do is to connect them e.g. through ADAT and then to route channels between units as required.

From operational standpoint and more freedom in terms of TM FX routing it's better to get a recording interface which supports the needed amount of channels / devices so that you can route any HW input and SW playback channel to any HW output (aka "submix").

It would be easier to post the specific video URL than more paraphrases that don't get to the point.

I'll just tell you the way I see it:

If your computer is powerful enough, you can usually connect up to three recording interfaces (sometimes more) to your computer.

You either connect them in parallel, each FF800 to its own Firewire port on the computer, or you form a chain of devices behind one firewire port on the computer. As far as I remember, it might be best to use Firewire 800 for the latter. I personally would prefer, that each device gets it's own firewire port.

The devices must be clock synchronised, either via WC (word clock) or by using digital ports / connections for clock transmission (ADAT, SPDIF, ...). Only one unit can be clock master (driver settings: clock source "internal"), the other units need to be configured as "clock slave" (driver settings: clock source e.g. "ADAT"). So WC is not necessarily required for such a setup, but has advantages if you intend to use the digital ports of both devices for other purposes.

When using several devices under Windows, make sure that you set the same ASIO buffer size for each device in the driver settings, otherwise you will get a slight offset in the tracks of the different devices when recording.

If you connect the devices in the way described above, i.e. each device has a FireWire connection to the computer, then you can access all I/O ports with the application (DAW, ...) via the ASIO driver.

If you route via the DAW, this is unproblematic, but then you always have the full RTL latency.
If you want to route on the card (FF800), then the routing only works within an interface, you cannot, for example, route an input port from device 1 to an output port on device 2.

In addition to the above-mentioned connection, there is of course always the possibility of connecting a second recording interface via ADAT. In this case, however, you no longer have a single port available; you can only have up to 8 additional analogue inputs and outputs via ADAT at single speed. You are free to operate the second device stand-alone, if this is supported.

In your case, it would be more advantageous to connect both devices in one way or another (2 devices parallel to each other on 2 Firewire ports or serially one after the other on a Firewire port on the computer) in order to be able to access all ports.