I don't know the SP-6's layout. I strongly suspect that it's just another tape out by a different name instead of Tape 1/Tape 2. I'd just about guarantee it if you only have a single tape input and a single tape output. In such case, the monitor out would allow for tape-to-tape recording by allowing a second tape recorder to get an input signal via the monitor out jacks while the tape input/output jacks are occupied by the source tape deck.
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