If the Saffire Pro 40 has a standalone mode with the possibility to store routings (analog in to ADAT out and vice versa), then it would be possible to use it as an extension unit. You can search for the manual and find it out (or wait for someone chiming in here knowing more about the Saffire).
There is also another possibility.
It should be possible to fully install the Saffire Pro 40 with driver and management software, you need it anyway at least one time to configure the necessary settings and the routing.
Then I would keep the Saffire Pro 40 connected for a while this way because then it is still fully manageable.
Once you are confident, that you can live with the static settings, look how you can store this permanently into the Saffire Pro (it might remember last settings, not sure, check manual).
Then remove the Firewire cable if both devices are turned off.
Firewire 400 plugs can destroy electronic during plugging/unplugging if the plug tilted / not pulled straight (without "angle").
In the DAW or in audio programs, simply load the RME driver and all is fine, the Saffire driver will not disturb.
Under Windows, check, that the Windows default audio device is pointing to the correct RME WDM device.
The same should be possible under Apple.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13