I am stuck in the same boat, trying to calculate the determinant of transfer function matrices for the purpose of checking the MIMO Nyquist stability criteria, see MIMO Stability ETH Zurich or Lecture slides (pg 10). Unfortunately there does not seem to be a simple MATLAB command for this. I figured it can be evaluated manually.
If you have a 2x2 transfer function (TF) matrix G(s) of the following form:
G = [g_11 g_12; g_21 g_22];
you can obtain the determinant by evaluating it as per its original definition as
det_G = g_11*g_22 - g_12*g_21;
This will result in a 1x1 TF variable which you can use for further calculations. The only problem occurs when you need to evaluate it equal to zero to find the roots for any reason. Then you need to use transform this variable det_G manually into a symbolic term using the symbolic toolbox.
s = tf('s');
L = [ 2/s/(s+1) , 1/s^2 ; 0 , 1/(s+2) ];
I = eye(2);
G = I+L;
det_G = G(1,1)*G(2,2) - G(1,2)*G(2,1);
s^3 + 4 s^2 + 5 s + 6
s^3 + 3 s^2 + 2 s
If you now want to find its roots, execute
det_G = (s^3 + 4 *s^2 + 5 *s + 6) / (s^3 + 3 *s^2 + 2 *s);
sol_s = solve(det_G==0,s)