As I said before, I don't own the US-2400 and I'm not a Mac person so I unfortunately can't give specific detail. I'm fairly confident it should work based on my first hand experience and other's reported success on Steinberg's forum ( I see you asked the same question there as well
).
So generally speaking.....
Drivers:
If you already have the US-2400 installed then your drivers are already installed as you are just adding another of the same device. This is how Windows behaves and I would think it would be the same for Mac. I'm not sure if specific USB drivers are required. There's a software LCD installation package for the unit but I'm not sure if it installs just the application or if it also includes specific drivers.
Cubase Settings:
If you've already have the 1st US-2400 configured in Cubase then you'll just need to add 3 more Mackie Controllers in Cubase's device setup using the additional USB MIDI ports that will be listed once you connect the 2nd US-2400. The names of the US-2400 USB MIDI ports are different than when installed in Windows. When I set it up on my friend's Mac the OS named the ports something less than obvious. In other words, it was trial and error until I got the right ports and mapping...but it worked.
I also remember in addition to the 3 USB MIDI ports (for the 3 banks of faders), there were 2 other Mackie controllers I had to add. One for the Jog Wheel and one for the Directional Joystick (used for Surround Sound control).
If you do attempt this and get it working, please document for the forum's archive. Good Luck!
