Bistatic and monostatic RCS is a bit different in the sense that bistatic RCS not only depends on the incident angle, but also depends on the departing angle. If the difference between the two angles are small, you can use the monostatic RCS to approximate the bistatic RCS.
What is your use case for modeling bistatic RCS? Do you have a measurement that you want to incorporate into the object? If so would it be possible to share the format of the measurement?