The driver will support multiple units, but three is the maximum that RME is testing. More is theoretically possible, other question is, whether your computer is capable to supporting that many channels over its USB infrastructure.
A workaround might be to install add-on cards with USB3 chips of their own.

I have the Sonnet card with 4xUSB3 (FL1100) which gives you a dedicated USB3 chip per connector/interface.
But it won't be produced anymore, it required 4x PCIe lanes and was expensive, around €150.
Now where the card is not available anymore, people tried to get €300 on eBay when I looked 2y ago.
But a spare for that price is a no-go.

Another thing is that there are more up-to-date ASMedia USB3 chips, which do not seem to have the issues of past USB3 generation chips. It was mentioned a while ago by MC, and I also made the experience that another add-on card with USB3 chips from ASMedia worked well in my setup. At that time, I think with even two UFX+.

You will (of course) have to work with an instance of TM FX per device.
There is no hw-backplane to connect both devices so that input from device #1 can be routed to the output of device #2.

So, you have to route through the DAW or to sacrifice ADAT ports to route at least a few channels between device #1 and #2. You could also use MADI.

DURec recording will work, but on one USB driver/stick per UFX unit.

Another thing, as UFX III has faster converters, you will get a little track offset when using together with UFX+.