Is there no 'Mix' position along with 'Stereo 1' and 'Stereo 2', where you can hear both the recorded sound and the new sound you've dubbed onto the tape? Failing that, you might have to run the tape through twice, once to capture the original audio and a second time to capture the dubbed sound. Try and leave dubbing sound until post production as it is much safer and you have the option of changing it or deleting it.

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