When you compute and save the Bandstructure object, both spins are automatically included. So to extract only one spin components, you do as you mentioned "evaluate(spin=Spin.Up)". Like:
bs = nlread("file.nc", Bandstructure)[0]
bs_up = bs.evaluate(spin=Spin.Up)
bs_dn = bs.evaluate(spin=Spin.Down)
After that it all depends what you want to do with it...