'Shouldn't both drawers just open simultaneously when hitting the eject button?', no.
Use '
Ardiem' to configure a key to open and close the CD-587's tray.
With or without 'Ardiem', open the (now configured as a 'slave') CD-587's tray, insert a CD, and close the tray - does the CD's icon on the 'Desktop'? If not, try the test below.
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You still have not listed specifically which PowerMac G4 model you have.
As a test ...
Turn OFF the PowerMac G4.
Remove the DVD (DVR-107D) and place it aside.
Remove the Matshita CR-587 and configure it as a 'master'.
Insert the CR-587 into the upper bay (where the DVR-107D originally was)
Turn ON the PowerMac G4.
Place a CD into the CR-587's tray and close.
If the CD is recognized and its inserted CD's contents viewable, then you confirmed your PowerMac G4 only allows a removable media device (CDROM, CD burner, DVDROM, DVD burner, etc.) as a 'master' only.
Report your observations.