Pin 25 is a supply rail for termination power. Both Macs should have identical voltages to prevent power supply problems.
Look at this pinout:
http://pinouts.ru/HD/ScsiExternalAmigaMac_pinout.shtml
You must consider data directions. Any item with an arrow pointing to the right is an OUTPUT from the Mac. These signals change very quickly, so hooking two Macs together will result in many of these signals conflicting simultaneously. One Mac attempts to output low, the other attempting to output high.
You will VERY LIKELY BURN your SCSI chips if you do this for very long. Maybe instantaneously. The Macs will still work, but will be limited to booting from floppies with SCSI blown.
Maybe a modified idea could satisfy your curiosity. Take an external SCSI drive. Boot Mac A off of it. When it's done booting, pull the SCSI cable and plug it into Mac B and boot off of it. Pull the SCSI cable and switch back to Mac A. etc. You will screw things up this way and have to reformat the drive after a while, but you will not damage any hardware.
Look at this pinout:
http://pinouts.ru/HD/ScsiExternalAmigaMac_pinout.shtml
You must consider data directions. Any item with an arrow pointing to the right is an OUTPUT from the Mac. These signals change very quickly, so hooking two Macs together will result in many of these signals conflicting simultaneously. One Mac attempts to output low, the other attempting to output high.
You will VERY LIKELY BURN your SCSI chips if you do this for very long. Maybe instantaneously. The Macs will still work, but will be limited to booting from floppies with SCSI blown.
Maybe a modified idea could satisfy your curiosity. Take an external SCSI drive. Boot Mac A off of it. When it's done booting, pull the SCSI cable and plug it into Mac B and boot off of it. Pull the SCSI cable and switch back to Mac A. etc. You will screw things up this way and have to reformat the drive after a while, but you will not damage any hardware.