LapLink Macintosh III from Traveling Software. It works only on the Macintosh Portable since the SCSI ID of the Macintosh Portable can be changed from 7 to 6, avoiding the ID conflicts. Traveling Software also provides a special SCSI-SCSI cable, which is required for this connection. (This setup cannot connect two desktop Macintosh systems unless the drive ID can be changed. This is not possible with Apple's internal drives but you may be able to do it with other internals. In any case, this is unsupported, so if you attempt this you do so at your own risk. You probably will void your warranty.)
LapLink Macintosh III
Traveling Software, Inc.
18702 North Creek Parkwaybr
Bothell, WA 98011
206-483-8088