If you need double speed only sometimes, then I would organize it this way, that the most needed devices are connected to the first Ferrofish converter. Then you can use everything connected to the 1st converter at double speed.
If you need only single speed, you can access the channels for all device at once.

See MADI simply as a highway for audio data with usually 64 channels.

For completeness only (not applicable in this case for UFX III): there is also a 56 channel format if pitch is supported by the driver. But this is not applicable for the MADIface USB driver.

You can connect one or chain multiple devices in a serial fashion as I told you already.
Each connected MADI device has access to all 64 channels.

If you intend to use double speed, then configure the 96k frame format in the driver settings and on each of the MADI devices. Then you can make the UFX III to act as clock master. All slaves can then use MADI for clock synchronization and can by this switch automatically between single and double speed based on the project settings in the DAW.
As the DAW loads the driver then the driver will instruct the UFX III which sample rate to choose and the UFX III as clock master will send the clock out on each of its digital output channels. The slaves will follow the master / the application/DAW.

If you use double (or quad) speed, then two (four) channels are being used for channel multiplexing to achieve the higher bandwidth that is needed for double and quad speed.
Therefore, you have in double speed only 32 and in quad speed only 16 of those 64 channels.

The forwarding of audio packets from MADI input to MADI output on each of the connected MADI devices takes only 3 samples at single, 6 at double speed, this is very low, so you can simply ignore that.

On each of the devices that you connect on a MADI bus you need to configure the routing:
- from which MADI input channels they take data for their inputs
- to which MADI output channels you send data of the devices outputs
Depending on the device it might be, that you need to configure also a routing to pass-through all the remaining channels.

Older RME devices following an 8-port schema had an automatism for that (RME MADI extensions, "Auto-CA").
For the more modern and 3rd party devices you need to configure the three mentioned routing items for each MADI device manually.

You can even chain one or more PCs to this MADI chain of devices, then you can use both devices for even making use of the recorded audio or to use them for backup recording. If you use Windows you can for example use Global Record (from the older version of DIGIcheck) which is a very reliable and "light-weight" recording application.

So, a PC in combination with a MADIface USB card plus Global Record can be used for getting similar functionality as with DURec to get a backup recording. Also here you would get for the internal MADI routing a little latency of 3 samples per additional device at single speed.

I hope this explains the possibilities a little bit more in detail.