I haven't seen 2 hard drives, but I have seen some extended brackets that were for adding a hard disk to a 2-floppy SE / SE 30. I expect with one of those and a bit of creative wiring, you could get 2 HDs in. You'd want both drives to be low power so you don't fry the power supply.

Errr... you did mean add internally? If an external HD is what you're asking about, any ol' SCSI drive enclosure will do. You just have to set the SCSI ID to something different than your internal HD.