Rharv has the right suggestion for you. Real Band or at least another sequencer like Sonar, Pro Tools, whatever. Biab is not designed for this. I'm not saying it's impossible, there's probably some clever workarounds but why bother when you already have RB? RB has a full mixer console just like you asked for, you can drag your completed Biab tracks or generate them from scratch from within RB, hook up your Nord and all your different synths, each on it's own track, and record as many parts as you want.
If you want to play your keyboard in Biab use the Thru track, select the Saffire midi driver and make sure you match the midi channel.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.