Can't say for slackware,
but Mandrake see's my dvdrom as hdb, but my cdrw as /dev/scd0.
Hence I'd imagine that all you need to do is to see what the hardware is recognised as.
i.e. if the first device is /dev/scd0, then the second one is gonna be /dev/scd1, but don't forget that there are a couple of different naming conventions, I believe it's something to do with drivers etc so if it's using scsi emulation it could be /dev/scdxxx or /dev/sgxxx or even /dev/srxxx
sorry if that's no help!