Thats exactly what i've been doing for the last 5 years. You need to make stereo recordings with each voice recorded to one side of the tape ONLY. To do so you'll need a multi-track deck. I used to do it that way, tho now i use a PC with cool edit pro software.
But If you want to do it with a tape, look for a 4 track cassette deck like a Tascam. You can find them cheap nowadays since PC's have surpassed them 1000 fold. What you do is set it to record on track 1, then record a voice only on that track. (you'll probably need to pan that channel, but thats something you'd have to learn as you learn to use it) Then you'd record the other skull's answer on the 2nd channel. Channels 1 and 2 on a 4 track recorder are generally the same as right and left on a stereo. They will then be totally spereated, IE: one skull's voice coming from the left speaker, the other from the right.

Anyway, it's really very simple to learn, and you can pick up a used 4 track as cheap as $50. But personally i would never do it that way if you have a PC.(which you obviously do since you're online...unless it's someone elses) You would just need some software like cool edit or some other multi track software and you'll be able to do so much more so much better it's rediculous.

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