I'm not too familiar with Sound Forge but I strongly believe you can just duplicate the figure 8 channel after you recorded it and flip the phase of the duplicated track, hard panning the 2 tracks accordingly. For MS it's recommended to use the same kind of microphone but it will work fine anyways.
The setup you mentioned is correct, you are picking up the side Waves with the figure 8 and the front with the cardioid. You can use this technique with anything (including guitar amps) and it gives you a great stereo image.
Let me know if it helped.