tested with octave 9.2.0 and control 4.0.1
in matlab, you can assign individual elements of a MIMO transfer function:

Matlab 2024a

>> A = tf(magic(2))

A =

  From input 1 to output...
   1:  1

   2:  4

  From input 2 to output...
   1:  3

   2:  2

Static gain.
Model Properties
>> B = tf(zeros(2))

B =

  From input 1 to output...
   1:  0

   2:  0

  From input 2 to output...
   1:  0

   2:  0

Static gain.
Model Properties
>> B(1,1) = A(1,1)

B =

  From input 1 to output...
   1:  1

   2:  0

  From input 2 to output...
   1:  0

   2:  0

Static gain.
Model Properties
>> B(2,2) = tf([1 2], [3 4])

B =

  From input 1 to output...
   1:  1

   2:  0

  From input 2 to output...
   1:  0

        s + 2
   2:  -------
       3 s + 4

Continuous-time transfer function.
Model Properties

Octave issues an error on assignment:

> A = tf(magic(2))

Transfer function 'A' from input 'u1' to output ...

 y1:  4

 y2:  1

Transfer function 'A' from input 'u2' to output ...

 y1:  3

 y2:  2

Static gain.
>> B = tf(zeros(2))

Transfer function 'B' from input 'u1' to output ...

 y1:  0

 y2:  0

Transfer function 'B' from input 'u2' to output ...

 y1:  0

 y2:  0

Static gain.
>> B(1,1) = A(1,1)
error: lti: subsasgn: invalid subscripted assignment type '()'.  you must select an LTI key first, i.e.  sys.keyname(...) = ...
error: called from
    subsasgn at line 49 column 9
>> B(2,2) = tf([1 2], [3 4])
error: lti: subsasgn: invalid subscripted assignment type '()'.  you must select an LTI key first, i.e.  sys.keyname(...) = ...
error: called from
    subsasgn at line 49 column 9

there may be some workarounds, but this sort of functionality is in some of the MIMO matlab help examples, so we should probably expect it to work.