A DAW is only able to open one ASIO driver. Since those 2 units are supported by 2 different RME driver
you can't open one ASIO driver in i.e. Cubase and then run them both.
ASIO4ALL is a hack, I can't recommend to make use of this to connect them both,
as then it runs based on WDM and the Windows Mixer is in the signal path (not pure ASIO).
I remember somebody said on this forum that Steinberg itself has a trick with an additional driver or smth like this.
If I were you I would connect the FF400 via ADAT to the UCX.
You can connect both interfaces to your PC and then control the routing of both interfaces.
In FF400 route the wanted channels to ADAT OUT and connect it to ADAT in on the FF UCX.
In Cubase you would simply load the UCX driver and access I/Os from the FF400 via ADAT.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13