I don’t know about the Marantz, but I have not had good results with the "bi-amping" options in these processors. Most of the time, they do not truly output the same signal on both "biamp outputs". They have a circuit that examines the actual waveform used by the main outputs and then sends it through a "comparator" circuit to subtract those waveforms. It then takes the difference and send it to the optional "bi-amp output". The result is highly reduced sound quality. It is much better to use a splitter Y-cable. However, you could possibly have other problems to deal with like almarg suggested (mixing input impedances of multiple amp channels). This can also result in reduced sound quality (weak bass, other). It may be just better not to biamp at all and run the speakers from one amp channel.
If you really want to do a bi-amp splitter cable, you can look at this one, in addition to almarg’s suggestions:
It’s very high quality OCC copper, but it does use the silver-plated Neutrik XLR connectors, so keep that in mind if you are looking for a particular sonic signature.
