Welcome!
I think this is how you might be able to get that to work.
You can create two separate MIDI in blocks, routing one to each sound you want to use. you can then split the keyboard range in each block.
Then, create a dial widget, assign it to the MIDI In block for the bass sound. Scroll through the parameters to MinNote or MaxNote. Set the widget value as you see fit.
When you come out of Edit mode, turn the dial widget.
You’ll see the note values change, and that will change the MinNote/MaxNote(whichever you’ve chosen) of the split.