User`s guide

3 Operations on LTI Models
3-10
For state-space models, both concatenation and subsystem assignment
increase the model order because they assume t hat
sys and h have
independent states. If you intend to keep the same state matrix and merely
update the input-to-state or state-to-output relations, use
set instead and
modify the corresponding state-space data directly. For example,
sys = ss(a,b1,c,d1)
set(sys,'b',[b1 b2],'d',[d1 d2])
adds a second input to the state-space model sys by appending the B and D
matrices. You should simultaneously modify both matrices with a single
set
command. Indeed, the statements
sys.b = [b1 b2]
and
set(sys,'b',[b1 b2])
cause an error because they create invalid intermediate models in which the B
and D matrices have inconsistent column dimensions.