TYPICAL DUAL CASSETTE DECK: The first transport is a playback deck only. Its output is recorded on the second transport only. It doesn´t have separate outputs which may be routed to the XR® 600E (or any mixer). The second transport has inputs and outputs which function like a single cassette deck. Even if the tape inputs on the XR 600E were routed to the XR600E tape outputs, it wouldn´t be possible to playback on the first transport and record on the second transport.
NOTE: If the dual cassette deck does have separate inputs and outputs for each transport, then it is possible to record from the playback deck through the powered mixers Tape in/Tape out on the XR 684 and XR 600F to the record deck.