Refer, again, to the Owner's Manual.
Many receivers of that age gave the option of either recording the source (phono, tuner, etc.)
or copying from one deck to the other --- usually #1 to 2, but not from #2 to #1.
Some went the other way, from #2 to #1, but not #1 to #2.
A few did allow copying in either direction,
and a small number could, to a certain degree, handle three decks.
A very handy accessory (I have a few of these)
is the Radio Shack 42-2115 tape deck switcher.
It allows one monitor loop on a receiver to handle three decks,
and you can copy from any deck to either or both of the other two.