Characteristic impedance of a transmission line
Above about 1MHz, normal cable characteristic impedance is fairly contant with frequency as per the equation: -

\$Z_0 = \sqrt{\dfrac{L}{C}}\$ where L and C are inductance and capacitance per unit distance.

The theoretically correct equation for all frequencies is: -

\$Z_0 = \sqrt{\dfrac{R+j\omega L}{G + j\omega C}}\$ where R and G are series resistance and shunt conductance per unit distance.

This tends to reduce to a complex value in the sub-1MHz area of: -

\$Z_0 = \sqrt{\dfrac{R}{j\omega C}}\$ 

If both frequencies are well above 1MHz then you can take Z0 to be a constant.