Ah the good ol' days. We had a Tascam 388 1/4in tape in our songwriting studio and I had a 246 cassette at home. I don't know of any right or easiest way to do what you want. There are track splitters now, I have never tried one but maybe you can save your final mix 2 track master cassette of your session to wav or mp3 and then try the splitter. Sort of a long trip around the bush for you but who knows maybe it'll work.

I used a little tape player/recorder and Cool Edit (Now known as Adobe Audition) years ago on a Dell XPS to save some old songs of ours from cassette to wav files. It did a decent job I thought.

Charlie Fogle might have some ideas. I'd ask him for sure if I were you.

Good luck.