Welcome to the forum.

There are already plenty of postings with clocking as topic where you could gather key elements of such a setup.
But anyway, here we go for your concrete setup.

Incidentally, Windows or Apple?

If Windows: the two devices do not use the same ASIO driver, so it's not possible to connect them in parallel.
Using ASIO4ALL is not a valid solution. Therefore, you need to chain them through ADAT.

[I am no expert in Apple, maybe there is a possiblility, check for aggregate device]

Either you preconfigure the FF400 on another computer with FireWire 400 for standalone operation, or you connect the Fireface 400 to a FireWire port of your computer. Then you can fully operate it, then you would load the USB ASIO driver of the BBF Pro in the application/DAW and access the FF400 ports through ADAT.

The physical setup (connect BBF Pro and FF400 with two TOSLINK cables):

PC-----USB2-----BBF Pro (clock master) ---ADAT OUT-------->-----ADAT IN------FF400 (Clock Slave)
  \                                                                                                                    |    standalone operation or
   +----FW400------------------------------------------------------------------------------+    operation through TM FX (2nd instance)
                                                                                                                             via FireWire ASIO driver
                         BBF Pro----------------------ADAT IN---------<----ADAT OUT---FF400  audio from FF400 to BBF Pro

You need at minimum one TOSLINK cable from BBF Pro as clock master towards the FF400, as clock signals can only be transferred on an ADAT output. But best would be to use two TOSLINK cables:
a) from BBF Pro ADAT OUT to FF400 ADAT IN
    to route audio from BBF Pro through ADAT to outputs of the FF400 and for transmission of clock signal
b) from FF400 ADAT OUT to BBF Pro ADAT IN
    to route audio from ports of FF400 through ADAT to analog outputs.

The configuration in the driver settings dialog for
a) BBF Pro as clock master: clock source “internal”
b) the FF400 as clock slave: clock source “ADAT”