By the way. The use of capital letters on the internet is synonymous with shouting. Do you want to keep yelling at the forum or does your keyboard also supports lower case letters? ;-)

Why shouldn't that be possible? You can see for yourself that the Apollo X6 has ADAT inputs and outputs, just like the UFX II. So you can connect them to the UFX II and not only do the audio but also do the synchronization with them. Alternatively, the use of Word clock would also be possible. Both devices have that too.

You can choose who should become clock master.
When the Apollo X6 becomes clock master and the UFX II learns the clock via the ADAT digital inputs, RME SteadyClock takes care of removing any clock jitter. Thanks to SteadyClock technology, RME devices in such a setup are ideally suited for use as "clock slaves". Then you only need a TOSLINK connection from the Apollo X6 (ADAT OUT) to the UFX II (ADAT IN).
On the Apollo you configure clock source internal, on the UFX II clock source Optical 1 or Optical 2 (depending on what you connect the Apollo X6 to.
Then all you have to do is route the preamp inputs of the X6 to the ADAT outputs in the direction of the UFX II.

If you want the UFX II to be clock master, then you either require word clock (which I would avoid unless absolutely necessary). Instead, I would also lay a TOSLINK cable from the UFX II in the direction of X6. Then you set the UFX II clock source to internal and the X6 you have to set to learn the clock via the ADAT input.

I am uncertain whether the Apollo X6 is capable of standby-operation, it might be that you need the thunderbolt connection to your Apple to remote control the device as preamp and its routing to ADAT.

As the Apollo X6 is for Windows and Apple (according to Thomann) I am not sure what OS you have.
If you have Apple: There might be other ways to use the devices, maybe by configuring an aggregate device out of UFX II and Apollo X6, in this case you can operate both in parallel and route through the DAW if required for e.g. monitoring purposes.

Please be so kind to add key information about your setup (your computer, OS, ...) into the forum signature, so that it is not needed to ask back if you forget to mention the OS etc ...