I've had to build a custom loom to link these things so they can chat and control each other.
Working out the pinning without skills in electrical engineering is a challenge and managing the rotations of pin numbers between the sockets and fronts and backs of each 8 pin plug leaves many opportunities for geometry fuckups.
the chain here is, mp3s on the dmp3a unit on the right, 8 pin loom over to the left hand DH100ix which is doing display and amplifier duties. then the front line out pair go off to my electric drum kit because it's the only thing i had lying around that could take the only cable i had and then feed the output to headphones which is where the audio falls out.